Hitchcocks in England offers an English made stainless steel pipe and free flow exhaust. Fits like a glove and takes the stock “puff puff puff” and turns it in to a “bang bang bang”! I would suggest that even though it probably doesnt make as big of a difference on a small single as it would with a big V8, but with my mechanical background, Id do it anyways. When adding a free flow exhaust pipe and muffler, you need to also install a free flow air cleaner such as a K&N filter. An engine is nothing more then a hi-tech air compressor. The more air you add to the engine (K&N filter) the more air you going to need to exhaust from the engine (free flow exhaust). Here is the thing… Now that you have added the filter and exhaust and have a better breathing engine, you now need to add more fuel to it. The more air you can get in to the engine and back out of it means that you need more fuel to compensate for the higher volume of air the engine is now getting. Other wise if you dont add more fuel, you could cause the engine to run lean. A lean engine runs hotter and can burn up the pistons. A stock GT 535 already comes in to the US quite lean so that it can pass the EPA regulations.
To make your 535 run at top performance and be happy, I would also install a Dinojet Power Commander 5 witch will allow you to add more fuel to the stock EFI fuel map. I in my strong opinion wouldnt add a free flow exhaust with out also adding a K&N filter as well as having a way to add a bit more fuel across the RPM range. Like I said, It probably doesnt make a huge difference with a small single compared to a large V8, but in my professional opinion, I wouldnt do one with out doing all 3. The 535 will start, idle and run better with a Power Commander 5 installed anyways.