I have sandybikespares stainless steel pipes with standard exhaust, and I had some issues with bike not revving past 7k or so (there's a thread here about it), and ended up replacing what appeared to be an ancient air filter with a K&N. No other changes. Bike now has more power than I ever wanted. It pulls away like a train and by around 10k the power is still coming on strong that discs in my dodgy lower back starts to protest from the sheer force of the meat-sack that I am getting pushed harder and harder against the seat.