SSJ4 Vegito

I personally think Fat Buu at Full Power (Against Vegeta) was the Buu who fought against SSJ3 Goku because, when you think about it Majin Buu never had to get angry again after he powered up against Dabura. Gohan went from "I'm sure I can beat this guy" to "OH FUCK I'm doomed RUUUN" Buu didn't get angry. Plus let's not forget even if Buu didn't get angry. That's not a big deal if Majin Vegeta with stand Buu's attacks (When he used that Awsome Explosive wave) then SSJ3 Goku couldn't be harmed too much by the attack. However Buu (When he's about to release Evil Buu) is different. I personally believe that Fat Buu (Full Power) is equal to Kid Buu since Kid Buu is unrestrained. Also if Goku didn't sense Evil Buu & Good Buu's battle from that world but did when Super Buu was born. SSJ3 Goku's power was sensed and since Kid Buu should be close to him, I would think he could be sensed too.
As for the battle with Fat Buu if Goku could truly only sense Fat Buu's power (What he used against Gohan & Dabura) then Goku wouldn't have gone SSJ3 after all he only was stalling for time. So he must have either sensed it or Buu was still at the level he fought Vegeta with. Also let's not forget Goku doesn't know about Buu's regeneration until his battle with him (That's probably why he was surprised) however Goku did deflect Buu's Kamehameha without any difficulty (he was surprised because, he learned it so fast). So I'd say Goku's statement about beating Fat Buu is perfectly legitimate.