Does it go in and out of gear with the engine off ?
If it's hard to get in and out of gear you may find the clutch plate or guide tube is bad, some plates bind on the splines and just need cleaning up. Which means it will need to be removed and checked.
As for you tuning 15% pulley, CAI are a good start just remember to go up a heat range on your spark plugs.
My advice would be to fit a better intercooler now you have a pulley, this will keep you intake air temps down. The standard ic will not cope in the heat.
And if you get abit of heat soak from your CAI it will compound the problem.
Best ic's out the IMO are : mini GP setup, GRS Motorsport version, Airtech.
If you want to run larger injectors if you could get hold of a set off 380cc of a jcw kit or equivalents then you can get a jcw coding done at mini
All this will run with the standard exhaust, if you have a standard rear section of the exhaust system you can do a "one ball mod" this will give you a liittle more pops and burbles and a little gain in power and less weight to carry around. And it's very simple to do.
Don't waist your money on a chip get a proper remap done by someone who knows the r53, the only time you need to get a remap done really is a big valve head and a cam depending on what you fit.
But don't get he wrong a remap will help but the gains will be small and it money better spent somewhere else.