My advice to you would be get down to a rolling road and find out what you're putting out already - you may find that the 189bhp superchip is a waste of time.
When I took my (standard) PD150 to the first golfgti.co.uk rolling road shoot out at AMD in Bicester (See here
http://www.golfgti.co.uk/viewpastevents.asp?eventID=42) it was pushing out 186bhp. The PD engines are notorious for exceeding the manufacturers claims, so much so than very few of them actually put out less than 165, most over 170bhp, so an aftermarket stage 1 remap to 185ish bhp is often not great value for money.
Find yourself a tuner that deals with diesel cars and has a rolling road, so you can get a curve done before and after - this will ensure you're getting tangible results for your money. You can also discuss re-circ/dump valve options with them. AMD would be a good choice (not sure of your location).
Either way, I would say, looking for 245+bhp you're going to be looking at exhaust, filter, remap, and most likely a hybrid blower, which is going to cost you at least £2k. Additionally, some of the boost hoses are fatally weak on the PD engines, and at 3-4 years old will be getting rittle and towards the end of their usable life, especially is you start shoving major boost through them - additional expense here. I waould estimate 250bhp is going to leave you no change from £3k and the car will still handle mighty wooly!
Have you thought about a suspension upgrade? It might not make you any faster in a drag race, but you'll be considerably quicker cross country and will cost you 1/4 of the money! Having gone from one of these to a car (not that much quicker in a straight line, granted) I can't over emphasise the value of having a car that handles well.
