A 351 will make your gps speed as fast as your previous speedo speed!
It enables you to run higher gearing. Before my 351 cc 117 KM/hr was my highest ever top speed on GPS (72/3 mph). I'm about 200 lbs though and that speed was with my 56 liter givi box on the back.
I spent much time on our nearly perfect toll hiway with the throttle wide open... seriously I've had the throttle wide open for probably 1/2 an hour straight. Glancing down at the GPS top speed would be between 105 and that one time 117. That 117 did NOT include me leaning over to get better wind resistance.
Pre 351 top speed was often to be had in 5th gear rather than 6th gear.
I did 351, pumper carb, pipe, air box mods all at the same time and haven't been satisfied with my jetting. The huge difference would the 0-60 time.
I actually recorded it by taking off my 2 double take mirrors and putting a camera in one mount aiming at my gps in the other mount with the gps screen set on big numbers; current speed, time, distance. Unfortunately I lost that though it was tough to read.
I know cycle world is going to beat my times - these times are just for comparison sakes. Cycle world has 1. smaller guys and 2. give it 100% as they don't care about breaking stuff as much as I do.
I on the other hand gave my test about 90% (No reving the engine/dropping the clutch), had my huge givi box on both times and of course am bigger.
Pre 351 was about 10 1/2 seconds to 60 mph and post mods was about 6 seconds. That really is a huge difference! (Here in Malaysia 10 1/2 seconds puts you faster than about 50% of the vehicles off the line. 6 seconds and you are faster than 98%.)
For safe cruising speed I like to have "a burst" above what I am cruising. Ie if someone is moving in on me, hitting the gas and pulling ahead is an option. Now I can cruise at 105 -110 and still have that "burst" if I hit the gas. Before there was nothing at 105; meaning full throttle in top gear at 100 mph would only move me very slowly up to 105 and then if I looked down 5 minutes later I might be at 110 or 115.
BTW those numbers I'm thinking are probably worse/more pessimistic than most simply because I believe our thick, humid, hot air is tough to go thru. Always 95F/85% humidity. That 117 by the way was in the early morning when it was much cooler.