leave the stock filter in there dude! i put 2000 miles on my kit beating the crap out of it non-stop with a stock, unmodified, untouched airfilter.
remember, if you put a better breathing flter element in, you will have to upjet. The performance increase will be 0 and you'll just have to worry about new jets and silly stuff like that.
explain to me how you have your carb setup... pics would help. everything is said abotu the stock filter might not matter considering you have a PHBG, i want to know how you have it set up...
Here is my new carb setup, this gave me no additional top end vs. the stock 12mm carb but i did gain some blips and revs and vrooms down low and in the mids.
Its a stock PA501 intake manifold with tons of dremeling to open up to 19mm. Removed the studs, hacked a stock puch exhaust header off, oh look! the puch header flange is the same bolt pattern as the honda intake mani flange! and its 19mm! lapped the flange flat again, used a stock puch exhaust gasket (yay for 19mm already!) bolted the header to the mani, went to THEZONE and found an amazing reducer to fit the 19mm header, and the bigger spigot on the carb. Clamp clamp, run new throttle cable to stock controls, no choke needed, drilled jet to 93, done. runs perfect!