Years ago I did do some testing, on some of my stuff & on some other riders quads.
What I found, say like you have a cylinder ported for the upper rpm ranges & it has that snap to it when it jumps on the pipe, that reed spacer will create more case volume & soften that hit jumping on the pipe just a small bit.
It take a rider with good feel though to tell the difference that it causes.
The spacers I used were the thick ones, like around 1/2 to 3/4" thick.
Another thing I found that will work on some engines, but not others. Theres a 38mm Keihin carb made for the 300EX Honda 4 poke that has an adapter made on to it, maybe like 2 extra inches of extention added to the intake side of the carb. You take that carb & mount it up on a fully modded national Yamaha Blaster engine & it will set that engine on fire. That carb by being longer, does like a ram effect to the intake charge & will up the HP a bit more.
Take that same carb & try to use it on a TRX or LT engine & you'll loose HP. That ram effect does not work on those engines.
We won a lot of races in the 200 air cooled class with that ol 300EX carb, course I had a good rider that could make it happen.
Like Eric Gorr said, Speed is 95% rider, 5% Horsepower.
Neil