The power arguments can easily be chalked up to them comparing his power as a SSJ1 which he used in a prolonged fight against Cell. Especially considering that Goku, who has no idea that Vegeta has SSJ2, does not refute his claim that he can finish Dabra off quickly - and it's never suggested Gohan's power is so inferior to his father's that he'd be weaker in the second form than Vegeta's first SSJ form.
It still doesn't explain how Vegeta was angry with Gohan's lack of success due to him slacking off, which, like I said, wouldn't make sense if Gohan just showed a form way stronger than they were seeing against Dabra. Rather, they'd be talking about how Gohan can't access SS2 anymore. Moreover, Goku said that Gohan should just be angry like he was in the battle against Cell, and that way he'd be unstoppable. If he was just SS1, making reference of the events in the Budokai would just be enough, unless, of course, Gohan was already in the same form he showed at the Budokai.
It also doesn't make sense for Goku to be completely oblivious at Vegeta having SS2 when Vegeta said how Gohan at the Budokai was weak and pretty much said them are stronger than Gohan now (read: this is before the fight against Dabra happening):
Chapter: 452 (DBZ 258), P1.1-4
Goku: “Gohan, it’s your turn next, but have you trained properly?”
Vegeta: “Unfortunately, it seems he got carried away in peace and didn’t do any significant training. Our powers are higher than his now…Though I suppose there’s no telling what would happen if he snapped and went into a frenzy… ”
Kaioshin: “S-so that’s why these 3 have such composure...In a pinch, they can put forth tremendous power, like Son Goku displayed momentarily…”
So we have two options:
1- Goku already knew Vegeta had SS2;
2- Goku thought Vegeta SS1 > Gohan SS2
Not to mention that Goku specifically exalted Vegeta's intelligence at the Cell saga, saying overcoming the SS wall shouldn't be that much of a problem. Do you really expect he wouldn't believe Vegeta achieved SS2 after seven years of training considering this?
Chapter: 368 (DBZ 174), P12.7
Goku: “Vegeta is a genius…He ought to be able to surpass the Super Saiyan wall, but…”
Dabra being a Cell-level form can still be the case if they refer to the Cell who fought SSJ Gohan and Goku 7 years ago rather than his post-zenkai power.
Even pre-Zenkai Cell (Kanzentai Cell) was above the Boo arc SS, so Gohan wouldn't be able to match Dabura's power at SS1. Dabura also saw Goku's SS transformation vs Yakon (with a strong aura) and its reading (3000 kiri), yet was still pretty confident in taking not just Goku, but all the three SS at the same time, which wouldn't be possible if he didn't hold a significant advantage over them:
Chapter: 451 (DBZ 257), P2.5-6
Babidi: "Thr... 3,000 kiri...!! H-how does an Earthling have 3,000 kiri of energy...?!!
Dabra: “3,000 kiri…That’s strange…There shouldn’t be any human race like that.”
Chapter: 451 (DBZ 257), P12.1-2
Context: Babidi is concerned now that Goku and co. have defeated even Yakon.
Dabra: “…Alright then, I know what to do…I shall take away your anxiety, Lord Babidi…I shall personally go to Stage 3. Then in the blink of an eye I shall fill Majin Boo’s ball with their energy, you’ll see.”
Chapter: 452 (DBZ 258), P2.4
Context: as he confronts Goku and co.
Dabra: “...It’s quite impressive that you humans managed to defeat Yakon and come as far as Stage 3. I could even call it miraculous…However, this will be as far as you go. You’ve got me as an opponent now…”
Chapter: 452 (DBZ 258), P3.1-2
Vegeta: “Hmph…Seems Babidi is really panicking, if you, his #1 man are already making your appearance…”
Dabra: “Hmph…Let’s end the pointless blather there and have all of you come at me, at once.”
Still, Gohan held a sure advantage over Dabra, as stated in a guidebook named Dragon Box and implied when Dabra couldn't significantly harm Gohan (in fact, Gohan went apparently unharmed) and we saw how much Vegeta and Goku were dealing damage to each other in a battle of equals.
You can keep claiming the art is in favor of Gohan being SS2, but the story clearly goes with a different route.