I think V just said that because Urek was an unknown at the time. He isn't fully unsealed from Bam yet, he flew at a pace he figured was at least much faster than the rest can keep up with, it was even faster than all of them(Urek, Traumerei, Gustang) could see as he effectively blitzed all of them. His real surprise came that Urek can at least catch up to him, even if it's obvious V isn't going all out yet. So perhaps that and his ability to block attacks made V feel that he isn't quite sure of the scale of Urek and he can't be sure if he can win or not. I think once he starts to look more into it, maybe get some info from Luslec that fought Urek before he can get a better scale of things. We already know that sealed-nerfed V already outputs enough damage to make even Urek think that it's enough to kill a Family Head.
I'd like to think that Urek just has raw power on his side, but someone like V has a lot more experience where he can achieve better results while giving less. His entire shinsu(tension) is even built around that, the most damage with the least effort, that however doesn't mean he can't do more damage with more effort too.
I'd like to see full powered Luslec for a real fight though.