I think the most important thing to really take into consideration is that Goku's statement regarding Dabra's strength vs. Cell is, given the way he explained it, solely based on his movements from when he took out Kibito. It wasn't based on ki that was being sensed or anything like that, but just where he was judging Dabra's strength from what few actions he had seen beforehand.
Vegeta: “Hmph…The way things are going, this ‘Majin Boo’ guy isn’t going to be anything special, is he? Just like that ‘Dabra’ jerk…[ ] I’m saying that this ‘Dabra’ guy doesn’t seem as bad as you two feared. Looking at his attacks and movement outside earlier, it seems that if we just watch out for his spit, then we should manage something. I can’t call Kibito anything but a bumbler for getting done-in like that…”
Kaioshin: “…Is-is that true, Son Goku?...”
Goku: “Yeah…Well, even if that wasn’t him at full force…I think that before he would have been a frightening opponent, but…7 years ago there was a guy called ‘Cell’…[Dabra]’s probably about as strong as him…”
Additionally, his comment on Dabra being "stronger" than what he thought wasn't in relation to normal strength ki levels, but in his use of magic, something that Goku and the others aren't really familiar with fighting against.
Goku: “Magic, huh? He’s way stronger than I thought, ain’t he?”
So, in the end, it ends up being a situation of maybe Goku's estimate was just too generous. He's only taking a stab in the dark basically based on actions and movements, and not solid ki detection, so Dabra's actions might have led Goku to believe he was stronger than he actually was, hence why Gohan ended up being able to do as well as he did against him as a regular Super Saiya-jin.