I am running the SLP exhaust system and am very happy with it. Decent weight loss, not to loud and well made. I have also heard good things about the speedwerx set but have not had a chance to ride one. I ride 0-5,000 and rode about 2 tanks of gas with just the exhaust system before I added other mods (intake/head). I did not need to add fuel, but you will need to change your primary weights to pull the recommended RPM's (you will not get the full potential of the pipe unless you pull 8100-8200 RPM's). SLP has clutching recommendations for different elevations on their web site listed under the install PDF for the exhaust system. ***Caution***There is quite a bit of information on here about running the SLP exhaust system and some have needed to add fuel due to stock fuel pressure set point. I am running 46 psi and plugs/piston wash said I was still rich, but I believe that if you are at the bottom of Arctic Cat's spec (42 psi) you may need to add fuel. I would recommend having it tested so that you know where you are at. Great machine, I love mine.
Cheers,
Polarbear