i just used the 2 outside ones from graves on my bike. you can defintaly feel them. the bike will pull alittle harder up top but what you really notice is the power band is longer and has a noticable curve to it. also top speed will go up alot, before the stacks it would drop on its face and would creep up to 14,400 rpms with no headwind (close to 160mph, stock gears)
after the stacks it would do the same think but creep up to 15,500 rpms (close to 160, with 520 1 down 1 up) the 520 im sure helped pull that high.
anyway it also makes it easier to know when to shift without the light because stock the power delivery is real flat up top afterwards it has a real nice curve and you can really feel it top off.
another note, after i did this mod my gf wanted them on her r6 but i only bought 2 off ebay from some guy who had bought a set.
anyway i bought two short stack from yamaha, and it was like $4 for both.
its so easy to isntall the factory ones since there rubber just pop off the lid to the air box and pull them out and push the new ones in (the 2 center ones) takes like 5 minutes, no removing the air box.
her bike pulled exactly like mine did, no difference what-so-ever. i never got either dynoed but from the seat of the pants they felt identical, and they pulled to the same rpm's and her bike would literly pull to about 15,200 with stock gears in 6th! im sure thats close to 170 mph! no doubt would pull to 15,500 like mine di with different gears.
if i were to do it again i would have just thrown to factory stacks in mine.
sorry for the long post, but since i got to try both i thought id share.