Diesel Tuning Dude
Join Date: Jul 2007
Car: 75 Tourer Conn SE & 45 TDi VNT
To put it very briefly, the 'usual advice' for a 25 diesel for more horsepower would be (generic advice follows):
service > fit sdi injectors > increase boost (~19psi) > remap to suit
(+ decat if desired at any stage for slightly improved spool)
The remap and sdi injectors are where most power gains are to be had, you can do one or the other, or both but you must do at least one for more power. The decat as you mentioned wont do a right lot but will help with spool.
Other options like mafless, egr deletes etc are possible with a remap so if anything like this is necessary it sort of answers its own question.