Before you spend any money take a serious look at ESL chips and scoobyecu chips.
I have a 1993 WRX wagon with a TD05. When I got it, engine was completely standard with a TD04 running 0.75 bar of boost. Was nippy but not 'fast'. My turbo started macking an awfull noise 1 year after I bought it so I changed it to a TD05 with an ESL chip. This transformed the car as it now pushes 1.1 bar of boost and makes about 280BHP (It came de-catted). Now its certainly a lot faster.
However there is a noticable lag with the TD05 over the TD04, and ESL also do chips for the TD04 which raise boost to 1.0 bar and pump out about 270BHP. They cost about £195 I think (check ebay as they are always on there) and you just solder them into your existing ECU (easy job).
It really is the cheapest way of gettting another 40BHP. Personally I'd start with one of these then think about spending >1.5K on getting an extra 20BHP when you have just gained 40 for £195.
If I was making the choice again I'd go TD04 with an ESL chip ahead of my TD05 any day of the week, as the TD04 is very responsive.