Thanks very much for your help everyone.
I've done some number crunching and although the Porsche Boxster 3.2 S is affordable, it is probably not going to be feasible as a replacement car - it will be cheaper to tax than my RX8 and it will use a bit less fuel, but I'm concerned the running costs may run away with me (especially where part replacement and labour are concerned).
However, I am open to the idea of potentially test driving the Boxster and I still want to test drive the TTS and the Polo GTI (as I haven't given the Polo GTI much of a chance yet).
I mentioned earlier that I didn't find the Ford Focus ST seats very comfortable when I test drove one, and these are also used in the Fiesta ST - a shame really as it seems a good car.
The MX5 is a worthy shout. A good all rounder - and a very nice drive too - but the boot space is smaller than I'm used to. Perhaps I can overlook this since I'm not a family man; however it was the boot space which swung me towards the RX8.
Track day wise, I only tend to go on the track 2-4 times a year anyway, it's great fun in the right sort of car, but not a deal breaker for me. I'm more concerned with tax savings and fuel savings this time around.
Argh! Decisions decisions!