As far as weight and power goes, it depends how much you want to modify the M3 and how liveable you want it to be on the street. A full catless exhaust, pulley, drop in filter and tune can net you some impressive whp gains (
Click here). You can also lose quiet a bit of weight if you have the money and can live with the compromises. A light weight battery can save you 30-40lbs, two bucket seats can save 90-100lbs, an exhaust is about 40-50lbs, etc. You don't have to fully gut the interior to lose some meaningful weight.
Personally, I'd leave the M3 closer to stock (would probably get a catted exhaust and tune) so it can be enjoyed on the street without compromise since you have a GT3 Cup that is an untouchable track car