I went with teh stock tire high elevation kit and I only lost 1-2 mph on top speed. I am pretty sure with the over sized tire kit you will only loose 2-3 mph. I can only say buy the elevation that you ride in the most. All you need ot change out are the weights in the front clutch and that is the easiest part. Just be sure to torque the bolt to the correct amount and not to torque it against the belt. There is a lot of great info on the Nyroc forum about clutch kits. Just incase you wanted some more oppions.
Thanks
Dav