Turbobike Web Site www.Turborick.com
Stock gearbox with extra springs installed will be good for up to 300hp with no problems. Anything over I recommend a Lockup clutch as rick said. Mtc makes a snowflake lockup clutch that works very well on the gsxrs. Mre also makes lockup clutches but for only 86-89. Personally I prefer the mtc unit ,works very well!