You can expect your rpms to increase 67 rpm per 1,000 rpm in each gear at the same speed. Just say with your present gearing you are coming out of a corner in 3rd gear at 8,000 rpm, changing to 15-25, at the same speed, you will be turning 8536 rpm. It will give you more punch out of the corner, but you will loose a little top end down the straight. Are you getting into to 6th in the longest straight? Are you maxing out your rpms in this straight?