The jury is still out.
I have seen die hard land speed guys say that it hurt them.
I did some testing years ago and it showed a gain.
I can't remember the exact numbers, so i'll addlib.......
First pass was from second gear at 60mph, going thru the gears between 2 signs{on a closed course, ofcourse

}. The signs were about 9/10ths of a mile apart.
I went 182mph on the GPS.
Went back in 6th gear at about 100mph and rolled the throttle for the same distance.
Went I think 183mph.
Went home, did the small box mod and went back 2 hours later. Did the same 2 passes, and went 3mph faster on both.
The bike was a stock 2000 model with Yosh bolt on mufflers only.