Darren
I was very impressed with the KLX450 I rode a while ago.
Excellent engine and good ergos. I haven't heard of any problems with the bike and I've been keeping an ear out for them as it was such a good thing straight off.
I don't think a pipe is required for it as the motor has plenty in it as is, the best way to get a bike to go faster is to work on the rider
.