From tuning a few f20c with and without exhaust the conclusion is the factory OEM exhaust flows very well. I've seen the HKS HIFLOW (pm if you want to buy it, its done less then 500km) exhaust which has the same diameter piping as AP2 exhaust, slightly bigger piping Y pipes and round cannon like mufflers.

If you want noise then yeah go ahead and spend 1-1.5k on a aftermarket exhaust. The factory exhaust are well tuned and are capable of flowing with engine making up too 300kw atw (FI).