To be honest with you, I don't think you'll have a choice on this one. As you already know, you wouldn't be able to fit an OEM M3 muffler because the spare tire wheel well is in the way and all aftermarket M3 mufflers are built around the OEM m3 exhaust/fitment so even if you went with an aftermarket one, it still wouldn't fit unless you cut out the spare wheel well. The only thing you could do is fill the mid sections with resonators but even THEN it still might be too loud for your liking. As per you losing performance with the 90 degree angle statement, essentially you SHOULD lose a bit of performance because the airflow is changing path at such an extreme angle instead of free flowing all the way out the back of the car. Would it be noticeable? I have no idea, more than likely not though. Hope I helped a little bit.
BUILD THREAD SOON..IG @: ssblvck_