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Goku’s Prediction About Fusion

One of the most common arguments regarding how powerful Gotenks would become is Goku feeling confident about the strength of the product of the sum, and that it would be more powerful then himself or Innocent Buu. However many people fail to recognize that Goku’s confidence changed after important events took place.

We’ll start from the basics:

Chapter: 472 (DBZ 278), P9.4
Context: after explaining that Fusion lasts 30 minutes
Goku: “With these two, if they just manage to perform Fusion successfully, I think they’ll definitely be able to defeat [Boo] within 30 minutes. Fusion is just that extreme.”

One of the important factors to recognize is the distinction being made between Goku's change of words:

Chapter: 477 (DBZ 283), P11.8
Context: Chi-Chi thinks Goten will get killed by Boo
Goku: “Don’t worry. If he perfects the technique he’s trying now, he definitely ain’t gonna lose.”

Notice that Goku changes his opinion regarding the effectiveness of fusion from, “just managing to perform it successfully,” to “perfecting the technique.” There is a clear distinction being made here: why else would Goku finish his statement by implying that there is a possibility of Goten losing? In other-words, “If he doesn’t perfect the technique he’s trying now, he’s definitely going to lose.”

Argument 1) There is no distinction being made. All Goku states is that if they manage to fuse then they can defeat Majin Buu.

Counter-Argument 1) Incorrect. You would be forced to believe that managing to perform a technique successfully is synonymous with mastering it completely. This is a false interpretation of the statements being made.

Goku made his determination regarding the effectiveness of fusion before his battle with Majin Buu: in other-words, before he had an opportunity to gauge Majin Buu's battle power to its full capacity: and before Goten and Trunks displayed the upper-limits of their Ki.

Goku's battle with Majin Buu:
Chapter: 473 (DBZ 279), P8.3
Context: Goku confronts Babidi
Goku: “…Me and Vegeta both underestimated you too much…We never thought that Majin Boo would be so incredible.”

Chapter: 474 (DBZ 280), P13.3
Context: after Boo and Goku fight for awhile
Goku: “So he hardly took any damage?

Goku senses Goten and Trunks at full power:

Both of these events were the reasons why Goku changed his opinion about the strength level of a hypothetical Gotenks.

a) Majin Buu continuously surprised him with his strength, durability and capability of learning moves instantaneously.

b) Goten and Trunks, even as Super Saiyans, were not as powerful as he had hoped: they even attempt to scare him with their level of Ki but he remains otherwise unimpressed.

Furthermore, Goku had to ask Goten and Trunks whether or not they were at full power, which implies that, up until that point, he had no idea how powerful Goten and Trunks could become. Like-wise, Majin Buu's strength level surprised him, even at Super Saiyan 3. All of this tells us that Goku isn't mathematically trying to determine how strong the product of the sum would theoretically become, because because all he knows is that two weak individuals turned into somebody substantial:

Goku: “Those two from Metamor were completely weak and gentle on their own, but by using Fusion they transformed into a substantial warrior!"
Summarily, arguing that Goku's prediction is accurate is sheer hearsay. Two important events: which proceed the initial statement and/or Goku's initials estimations: completely change the foundation of Goku's logic.

Q1: But there is never a distinction made between a perfect and normal fusion?
A1: Yes there is:

Piccolo conforms to the ideology that Goten and Trunks only performed a successful fusion off chance, as opposed to getting the fundamentals down perfectly:

By Piccolo's own admission Gotenks Base Pre only appeared out of chance. Furthermore, Gotenks acquired capabilities that Piccolo was not aware of: he was able to become a Super Saiyan after fusing, which shows a greater degree of control then before-hand (the time that Majin Buu was busying pummeling him).
Chapter: 490 (DBZ 296), P5.3
Piccolo: “Oh! So he can become a Super Saiyan even after Fusion?!”

Q3: Is there any importance to those seemingly pointless “Fu” and “Sion” poses?
Toriyama: They’re incredibly important!
The series of movements leading up to “Ha!” are important for syncing the two people up and also concentrating their minds. But once you get really good at it, it’s also possible to merge with just the “Ha!”
Q4: Can anybody perform Fusion if their body size and power are the same?
Toriyama: It’s hard, but they can!
I think it’s possible to merge no matter the conditions, as long as it all goes well. I even think it’s possible for a larger number of people. However, since it’s quite hard for both parties to strike that delicate balance, it’s very difficult for two extremely different people to fuse. In that sense, synchronized swimmers would probably be really good at Fusion. By the way, there are many different types of Fusion, and the kind Goku learned is the one for increasing battle power.
Akira Toriyama has also established that there is a significant difference between the varying levels of control regarding fusion and its outcome. These are all influenced by a myriad of different factors. Conclusively, their greater degree of control, after training, implies that what they were doing was vital for the betterment of fusion.

This leaves us with one problem: Why would Goku risk the lives of his son and his family? Because, Goku previously established a back-up plan in case they failed:

As long as Piccolo and the Dragonballs were in tact then everything else was inconsequential. But that was not the only reason why Goku decided to take the gamble that he did, his moral principals and duties were still influencing his decisions at that point of time, and for a long time beforehand:

The Cell Games

The Majin Buu Saga
Goku: “No…I’m no longer a human who’s particularly even supposed to be here…I shouldn’t be the one to do it. It’d be better for these young guys to solve things somehow or another…After all, some other outrageous guy might show up eventually, right? …It's a nasty gamble, but…Seeing those two super-gifted squirts, it made me want to take this gamble…”

Goku: "However, I wanted the young guys to manage something…For the Earth’s sake too…”
We can now conclude that Goku initially felt confident about the strength level of a hypothetical Gotenks: his initial estimations were essentially inaccurate because they changed after a series of events: but later felt uncertain about their chances of success.

However, after establishing a back-up plan (the Dragonballs), and continuing to feel a large degree of influence from his mortal principals and duties, lead Goku to conclude that this alternative: as opposed to defeating Majin Buu himself: was worth the gamble.