Mark, it's good spec, but I would agree to replace stock airbox and sell your BMC seperately. The BMCs do well on ebay, but there is some "bad press" about them on s3s so it may put a few off.
The price you have is unrelaistic, I am afraid, that is your main problem. Forget the spec, that will seal a deal when you get people interested, but in the current market, you have the think carefully about price. Car is about 7-8 years old now? Car probably cost about £17k new.... A typical car in normal climate will depreciate about 50% in 3 years......obviously they will slow down depreciation after that..
If you want to sell it, you need to have it advertised at about £6250 I am afraid, if you wont take that for it, then keep it and use it. But at 100k plus, they do start to drop in value rapidly I am afraid.
You also have to look at what you are up agaisnt at that price, there is some seriously cheap metal out there now for 7k! And my mate was offered a brand new civic type R for £13550 at a dealer last week and a brand new clio 197 for £11495!...... Such factors will inevitably hit the 2nd hand hot hatch market.
If you have the original parts, going back to stock will always be more lucrative, parts will sell seperatley and originla car will shift for very similar money.
I know a guy who is selling a 53 plate, 60k miles, same spec as yours with sunroof, with 7k of IHI turbo, 1k of brembos and 1.5k of bilsteins on it for 10k this week, and he reckons he has doen pretty well to get that!
Good luck with your sale, it looks like a very well kept example and is a nice spec. But prepare for a fright if you really do want it gone!