Have a 300ex with a fresh rebuild. 11.5 piston. Stage 1 cam, full FMF exhaust, K&N filter. Would there be a noticable improvement putting on a 400ex carb and rev box. Does the 400ex use the 300ex stock cables? Also would be putting on a 350x carb boot.

 

I put a stock 400 ex carb on my 250x when I built it.I noticed it seemed alot more responsive off the clutch.Revved quicker and was quicker to wind down.I had no problems at all with the stock cables.Good luck I think itll be a good investment for you.