its not cheap, i've spent best part of 700 quid of rrps on performance mods, and only got about 10bhp to show for it. i've got a sports cat, full stealth system and a viper with a custom feed. Is much more fun to drive however. As you say no point in uprating the brakes as the 1.2 ones are more than adequate, and if possible remove the rear bench like i've done, makes a substantial difference, and sounds more sporty due to tehe enhanced exhaust noise
I'd spend your money on a set of coilovers, and have people in the twisties lol. Would def consider a viper as well, despite what most people say, its made a substantial difference on mine, just make sure its rooted properly.