Goku has seen the fusion in action when he learned it in the afterlife. He saw that, individually, the two fighters from Metamor weren't very powerful, but fused together they were formidable, and he could easily assess what would happen if Goten and Trunks fused together too.
Chapter: 469 (DBZ 275), P6.1-7
Context: after Goku laments that Gohan and Vegeta are dead
Goku: “I coulda used Fusion…”
Dende: “Fusion…! Merging together, right? That’s the specialty art of the people of Planet Metamor!”
Goku: “So you know about it, Dende…! That’s right, some people from Metamor who I met in the afterlife taught me that art…It’s a merging technique which can only be performed if two people are fairly close in both power and body size…In other words, by having two people merge into one, they’re able to become a single, new human with amazing power which either of them on their own absolutely wouldn’t be capable of. It really is incredible! Those two from Metamor were completely weak and gentle on their own, but by using Fusion they transformed into a substantial warrior! [ ] …I was just taught the art, but I ain’t never tested it out…There wasn’t anybody on par with me in the afterlife…”
Chapter: 472 (DBZ 278), P9.4
Context: after explaining that Fusion lasts 30 minutes
Goku: “With these two [Goten and Trunks], 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.”
Besides, it's pretty clear that Goku (and Piccolo thereafter) was planning for Gotenks to fight Buu as a Super Saiya-jin, not in their base form (as indicated by wanting them to get used to equalizing their Ssj battle power when he first had them power up). We see that Piccolo is impressed by Gotenks' Ssj level of power, and if it were below what Goku had indicated it'd be at, Piccolo's the kind of person that wouldn't hesitate to say it. Base Gotenks losing to Buu is one thing, but Goku and Piccolo were expecting him to go in fifty times stronger than he was then.
As for eight times stronger, since we know that Goku and Piccolo felt that Ssj Gotenks would be enough to destroy Fat Buu, and that Evil Buu was pretty much on par with Super Saiya-jin 3 Gotenks (and that the Ssj3 transformation is eight times more powerful than the Ssj transformation), Evil Buu would have to be over eight times more powerful than Fat Buu as well in order to be even with Ssj3 Gotenks's strength.
