If your after being able to handle what its already got then first should be the brakes and/or preferably a decent set of springs/dampers.
Most 'standard' 250bhp gta's go faster point to point when the drivers are able to actually drive them - adding another x bhp doesnt make any real difference
so maybe a driver training lesson is the best investment of all....
IMO the air intake system isnt worth anything more than a few bhp at most. A decent air filter to replace the paper one is most of the effect that an 'induction kit' gives (which is basically just a flasher version of whats already there, ie pipework and the standard stuff is alredy as large an internal diameter as the intake....)
If your after power - the best 'bang for buck' is definately a re-map.
Id tend to steer clear of the supersprint stuff - its hideously expensive and not a great improvement on the standard kit IMO (manifolds in particular), if you want a free'er flowing exhaust to help your remap then a pair of high flow cats and a new rear silencer generally does the trick - id stay away from 4 exit exhausts too - unfortunatley the gta exhaust sytem goes from the 2 manifolds into one pipe - so splitting it again later on i just a waste of money as it doesnt make any extar power over a single exit rear box.
A one off rolling road remap will of course be best as it tunes the ECU to your specific car, but the cost over going to say Autodelta for one of theirs isnt really worth the extra 10-15bhp you 'may' get out IMO.
so id say:
4. better air filter and reworked exhaust
if youve still got any cash left then get an LSD... of course if you have all the cash upfron then just get either a supercharger conversion or go the whole hog for AD's 3.7 GTAM set up