There are a lot of 'bolt on' modifications that you can do to an RST
here are some in no particular order
1. cam and vernier pulley change.. very easy
2. larger intercooler. very easy
3. Chip the ECU. easy enough if your confident with a slodering iron!
4. Port the head.. easy enough to make the holes bigger! hard to get it right to properly aid air flow though
5. exhaust & filter etc easy peasy.
6. install low compression pistons/ complete engine rebuild etc.. easy enough for the 'acomplished DIY mechanic'
7. replace turbo for larger item.. very easy.
8. change management.. requires a reasonable knoledge of wiring and engine management.

There are LOADS of things you can do you NEED a power figure to aim at so you can get the correct spec for that! no point putting a HUGE turbo on if you only want 200 Bhp