as for slip ons, they're ok but its only made for the stock header pipe. and if you want to get a header pipe for the slip on you have to
buy the complete system, pipe and silencer together, you'll get more HP from a full exhaust than a slip on

 

here are some results with the Full Alba pipe,jet kit, cdi box and Filter with max flow adaptor. Racing against 660 raptor with yosi slip on =tie, trx 450 with drd slip on lost by 1/2 bike lenght, stock yfz 450 lost by 2 bike lengths, dsx 650 beat by 1/2 bike lengt. 400ex with Procircuit slip on beat by 1 1/2 bike lenghts. I was really impressed with the way it performed that I sold My 400ex with the procircuit and Will be picking up my new Z400 today with a full Alba system on its way.