Sorry to dig up money's old thread but I was looking for his build thread and saw this. Couple of things.
1. Yes an eg coupe/sedan exhaust will fit. But ONLY a db7 or 99+ (pretty sure it was 99 but could be 98 or 00) db8. Any db8s between 94 and 97 have different length exhaust manifold, cat and midpipe compared to their db7 brothers. ALL 3 components are different, trust me. So if you go that route, keep that in mind and buy the appropriate parts.
2. An exhaust shop will make you a high quality 4.5" extension that goes between the midpipe and the axle back. I got mine in 2.5" pipe with nice thick piping and nice thick flanges and gaskets. I can make them for 60$. But I'm working on getting the price lower. I built mine way way stronger than I needed to.
Swapped my 96 dc2 greddy onto my 95 db7. Can post pics later, but like I said, all the lengths are different so I had to switch everything from the exhaust manifold back. The 4.5" extension flushed up the tip to where it belonged.
On the other side of that coin, if you have a gsr exhaust system you want on an LS, an exhaust shop needs to take 4.25" off at the cat end.
If you want to run an ls/type r exhaust on a gsr, you need a 4.25" extension. 5zigen makes one or an exhaust shop can fab one up.
Thought id contribute what I've learned in my research and experience. Also, all the same stuff applies to coupes on the ls/gsr stuff, just negate the 4.5" extension part.