hey what's up, long time no see.

well, you are not speeding the servo. the servo will always stay at .15s but when you punch up the expo% on your TX, you increase the sensitivity of the servo's reaction to your inputs = faster servo. to really feel it, try significant amount do not try 5% as you will not feel it in a buggy. try 0 and then change to +40% and drive it on the track, you will notice the steering changes.

cheers

Ali