Trust me, you don't need to worry about running a stock modded trans in a high hp bike. I run my transmissions in my 485 cheetah, 465 cub, a 612 twister, a 466 twister, and a couple stock cylinder 4mils, jrmarquez.702 is running one of my transmissions in his 400 cheetah and says it does good... You won't have any issues, and you can build it yourself....
side-cut the gears. (remove every other dog) (a local machine shop with a roatary table and an end-mill can do this for ya relatively cheap)
get a modded shift star
do the shift-pro kit, but re-use the stock heavy spring and chuck the softer shift-pro spring in the trash or sell it to someone on here if they want it.
do the shift-shaft mod (dremel out the shift-shaft so it can travel a little bit farther before hitting the alignment dowel.
get your shift drum machined for the second bearing on the end of the drum.
If you do these mods you won't have any trouble shifting. I have that same trans in my 10mil cub making 109hp and it shifts fine. backload it all you want, downshift, upshift, etc. Same transmission is in my 485 cheetah.
- Jared