I just recieved my new full exhaust system from Sommer KTM http://www.ktm-sommer.de/ for my 01 KTM 520SX. I bought the SPES MX system with the drilled air box cover and the needed main jets. I went for a ride on it yesterday and GOOD GOD!. The bike still has a linea power band with no real hits anywhere but it pulls much harder. It also revs much futher (but this be down to the air box mod). This mod makes the bike a MX only tool. To give you a better idea of the speed. My friend has a YZ 426 and at our local track there is a long uphill start(about 100 meters) I pulled a good 4 bike lengths passed him at the end of the straight. Before the exhaust change it was about equal. It is also lighter than the stock system. I was expecting the pipe to be much louder due to the visably larger muffler core. But to my surprise it was just a bit louder but nothing like a YZF after market pipe!. Fitting the pipe was not so easy and you need to have strong arms to release the springs. The main problem was that the pipe joining areas were so tight that I needed to sand them down and use a hammer to encourage them together!. But I feel that it was worth it.