I need help with Velocity Stacks!!!

Hey everone. It's been awhile since my last post, so here I go. As some of you out there may know I have a 636 with a MICRON!!! whhooo! sorry kind of an inside joke. any way, as well as some other accesories. I've been riding for some years now and as of this year on my new 636 I have found myself pushing the limits too many times on the street roads. So like any avid sport bike enthusiast, I'm gonna " Take it to the Track ". That being said, I'm very seriously going to put in some track time and lessons, as well as to try and become a better rider and possibly try to qualify at a race some day. This off season ( I live in Mn so I got 6 months of winter ) I'm gonna make some serious mods to my 636. New gaskets, cams, springs, airfilter, power commander, and VELOCITY STACKS. So, I found out what they are and where they go, but what do the do as far performance? Accelleration? Top end power? Smoothness and responsiveness? What? And the other thing is what are the differences in the different sizes that the come in? Should they all be short, should they all be tall, 1 & 4 short and 2 & 3 tall, vice versa, or doesn't it matter.

That is all, I appreciate you reading though this novel. Please let me know what to do and or who to talk to about these.

P.S. Kawai has some for sale here > http://www.buykawasaki.com/Default....ProductCatalog/default.asp?intCatalogID=1http
and Factory Pro has some Billet stacks here > http://www.factorypro.com/products/vstacks.html
why the huge price difference? do they really produce a difference of that much power?