Yes, it is going to reduce the speed and gas mileage. Effectively you are removing the 6th gear and putting in a new first gear. all the other gears are now effectively one gear down.
I know by this, it sounds like stupid change, but it does help.
I dont know about other people, but I can still get up to 143 MPH, which is plenty fast still. It is nice having the gear change due to me motor being in its best area (7k - 10k) a lot more of the time.
Yea, gas mileage goes down due to you not having a 6th gear, but I am not a heavy highway rider. still getting 33-43, mostly 33 because I can keep my damn hand from twisting....:rockon:
Waffles