From 20 to 25. The last 2 times to the track it was closer to 20 since I was driving into a 30mph - 40mph headwind and my car has the aerodynamics of an upright brick.
This discussion came up on another board i'm a member of... it seems most everyone with n/a or f/i cars gains from ~22-25mph no matter if they run a stock civic or turbo buick (myself).