I added a Ralliart muffler and gained around 40kms per tank, power also increased marginally. When i put my HKS exhaust on i actually noticed a slight drop in power. Could never figure that one out..
Well I've kept my car pretty much stock (engine-wise), and had a dyno before and after with just a fmic. It picked up 6 kws at the wheels. I'm also about to replace the rear muffler (diff back) with a Apexi one, so I'll see what gains there are from just that at the next dyno day.
Yeah sorry, I forgot to mention that. The before and after dyno runs were on the same dyno with the same weather conditions and same tyres. As about as controlled as you can get.