Hi all, thought I'd pitch in with my comments, despite not owning the car anymore.... (anybody seen it around MK or heard off the chap who bought it - Drew? I did point him in this general direction!)
Anyways, I removed front (replaced with Corbeau Sprints) and rear seats, all sound deadening (replaced the carpets) and the air-con condensor, but then fitted a 30kg half-cage. What to say? Well, I reckon the car was about 80kg lighter than factory, yet it was still perfectly balanced (measured on professional digital scales at all 4 corners), and behind the KW suspension, it was undoubtedly the biggest improvement I made to the car. Whilst the cage obviously added some weight, it also added some noticeable stiffness and of course, allowed me to use harnesses, which I would recommend to anybody doing track work as they make a MASSIVE difference to the driving experience on track and the confidence you have with the car. I'll admit however, all these changes did restrict the appeal of the car a little - i.e. it wasn't then ideal for a leisurely Sunday evening drive around the countryside, but that was not its purpose, so didn't bother me.
I guess ultimately it depends on what you want to use the car for. If I had my time again, I would still lose the reat seats and the sound-deadening, I would probably still fit the half-cage and harnesses too, but I would go for some slightly less compromising front seats and keep the air con. My view is that this way, you would have the perfect car for all occasions!