I have an eissenmann race cat back. Much lighter than the standard rear box and sounds great. Not too loud for normal driving and no droning at motorway cruising speeds. I was tempted by a de-cat to complement it but don't fancy the flat spot
Depending on how deep your pockets are, I've seen some fantastic titanium bespoke exhausts on Kumho cars. I think the company that makes them is called Zero - PHer houlbt would know.