Defo decat. Its my next mod on my st tdci. Just added a Cobra exhaust to my standard car and that made a nice difference, much more responsive! Decat is next when funds allow, the turbo will spool up a lot faster. People have had good results from universal intercoolers off ebay for under £100 notes!
With that mileage the car will benefit from cleaning the inlet manifold out and replacing all the seals. Run a can of BG244 through it to clear out the fuel system, STOC members swear by the stuff, and also consider adding some Wilkinsons 2 stroke oil to your tank to help lubricate the pumps etc. This will also quieten the engine down, make it run smoother and increase mpg.