Whats the difference between a UK spec 99 and the Sti. Obviously 215PS vs 276PS, is this a radically different engine, or just a UK similar in a higher state of tune.

I.e. for the ultimate is it worth uprating a 99 or should I have got an Sti ?

 

Check the FAQ at http://www.sidc.co.uk/faq.htm
this has all/most fo the differences between the cars. The STi is not a UK spec car in a higher state of tune, Its the equivalent of a UK car with a completely different engine, bigger turbo, different suspension etc etc etc.

If you want higher power, it would probably be cheaper to buy an STi, than to buy a UK car and spend a lot of cash on it. Especially once you go mad, as the warranty will be invalid, so if the engine goes, there is a fair chance you will not get paid out, or repaired.