Yeah, you will need to cut inthe spare tire well, add heat shields, etc. But if I'm not mistaken, the 323 isn't a true dual. You'll have to get a Y-pipe if I'm correct and should probably weld that on ( you can clamp but it looks half-assed that way, espcially it it's not a perfect fit...which i guess is fine if you won't show it and won't be under there too much). Depending on where you start the Y-pipe, you'll need new piping to the exhaust. To me, it seems like a lot of work just to put on an OEM M3 exhaust--especially since that exhaust has an annoying rasp usually(on the M3 at least, no idea what it would sound like on a 2.5). A decent alternative would be the Remus quad exhaust. It looks pretty much like the M3 from the back but has just a pipe running around the spare tire well so you don't have to cut and can retain your spare. Dunno how that sounds though.
The best alternative if you're willing to do all that work anyway is go custom. Cut off from the resonator back, you can reuse some of the piping. Get extra piping and you can route a pipe around the spare tire (you'll be doing a lot of welding). Then get glasspacks and weld those on (it's been done before, I can get a sound clip if you wanna hear it...it's almost like a UUC TSE that I have--and that UUC sounds sick). You'll have two brackets that used to hold the stock exhaust--the new exhaust will be light enough to be held by one bracket on each side so you won't need to buy a new bracket (saves you about 20 bucks haha). You'll have to weld little brackets onto each glasspack(that should be the easiest weld so far). Make sure to construct some heat shields for parts that didn't have it before but need it now. If you want it to be really loud, cut off the resonator and start farther back.
You might still run into a little complication depending on where the exhaust contours are on the bumper, but not as much as with the M3 exhaust.
To break it down(sorry all my prices are in American...but that's where I am):
Stock M3 exhaust: +-$300 USD on eBay, Y-pipe and other custom piping: 50-200USD, frustration of cutting through everything: priceless. Total: crica $500
Remus: +-$600USD and miscellaneous parts <$50. Total: circa $700
Custom: $60 for two glasspacks, piping: $50-200, renting a welder: $50, knowing that you have something truly custom and original: priceless. Total: circa $300 (can usually be done for less)