I haven't done anything to the air box and although I prefer a washable, reusable filter, I didn't even go there because the oem filter is really cheap...under $10.
I don't race either, but I always wondered how safe it is for the engine to ride above redline? When I look down and see I am over redline, I slow down (or switch gears) although I see people on race tracks riding well above redline. If you do mods to get additional performance out of the Grom, don't you have to reset the rev. limiter too? I already hit redline easily with my stock Grom, so I have been reluctant to try and get more speed out of it. What is the risk to the engine of riding say 70 mph consistently with a modified Grom? I know nobody can say for sure, but what is your estimate on engine/valve life?