My 330 sounded pretty good on a hot exhaust in Sport, but I nonetheless understand your desire for something that might accentuate it! Given the relatively limited options we have from BM direct (MPerformance Parts give you added trim options and exhaust tips but not actual exhaust upgrades, to my knowledge), then ACS is you're best option. All their mods appear to operate within warranty parameters:
https://shop.ac-schnitzer.co.uk/bmw-...0g21-791-c.asp