If the guy can run a 4.75, that's fast enough for the Falcons. Davis ran that, and basically a 4.75 is pretty quick for a DE. Now, of course guys like Peppers ran ungodly 4.5s coming out of school, but those types of athletes are rare.
Based on last year's combine results, the average DE ran about a 4.85. The average OLB ran about a 4.65. Basically meaning, I'd say anything less than a 4.75 is considered LB speed, which equates to fast for a DE. 3 of 12 OLBs that participated in the 40 ran a 4.7 or higher. Only 3 of 19 DEs that participated ran a 4.7 or faster.
The Falcons aren't really looking for size, but guys who can be like Simeon Rice/John Abraham, and makes plays vs. the run in pursuit. Davis has the potential to do that. And he's shown flashes of that ability already. I think the thing Davis needs to improve on the most in order to become a starter is 1) His recognition. He's caught out of position too much vs. the run to be a reliable 3-down starter. 2) HIs pass rush ability. The quickness is there, but he needs to use his hands better and get more moves if he wants to be more than a situational guy.
Dumervil if he can run really fast 40 will solidify himself as a 1st round, but he'll probably need to run a sub 4.65 to do that. Also look for him to be looked at by a lot of teams as a OLB in the 3-4 scheme.
"Vincere scis, Hannibal, victoria uti nescis" -- Maharbal, 216 B.C.E.