I agree with you the RS3 is hard to beat as an all rounder. I drove the new RS4 on road and track at the Audi launch event I don't think it offered much over the RS3, great V8 sound track of course, but it didn't feel much quicker and the performance figures seem to support that. If you needed even more practicality it would make sense.
RS3 vs RS5, I'd keep the 3 because I like the turbo power delivery and overtaking ability, I think you'd have to work the V8 harder for the same result and I'm sure the running costs would be higher. The RS5 wins on looks of course but it does feel a much bigger car on A & B roads.
The GTR, 550bhp and a warranty, is the car that calls to me but it's not a car I'd want to use daily so it adds all the complication of a commuting car to go with it. The running costs are almost double the RS3 too.
Strangely, the other car I still like is the Renaultsport Megane 265, great fun on track with low running costs and still practical too. Yes the downside is questionable build quality etc but then the RS3 has hardly proved fault free for many. Less money spent buying the car, more to spend on track day fun...
As the GTR option looks expensive the idea I'm taken with at the moment is a Caterham Supersport R or Westfield Sport Turbo (both good for road and track work, I don't want one I've got to trailer about) to go with the RS3 - I did the same when I owned a TT quattro Sport, best of both worlds.
One thing putting me off changing the RS3 at the moment is the trade in values. According to the latest CAP Automotive figures in the Telegraph, the RS3 is 17th best car for retaining value at 63% after 3 years and 30,000 miles (about £25k from list not including options). That seems to be at odds with the current Glass's and similar sub £30K part ex prices, either that or the RS3 has done most of it's depreciating and running it for the next 18 months becomes much cheaper.
Top 20 cars that hold their value - Telegraph