Yep, DEs like DTs typically take 3 years before they reach potential. If the Falcons draft a DE this year, he's not going to change much about the team's pass rush in 2011. It's an attempt to solidify the pass rush for 2012 and beyond.
But even still, only about 30-40% of DEs turn to into what most would call good pass rushers that can consistently get 6-8 sacks/yr., let alone double digit sack guys.
This is why I liked the idea of a Charles Johnson in FA, and then a 1st or 2nd rounder if they don't feel like Sidbury's coming along. You get the solid player at LE, AND then the guy who'll hopefully be ready to be an 8 sack guy when Abe leaves.
This offseason's very annoying.