Well these are the options I know of for retro-fit bumping the power in a NA Carrera:

1) Powerkit (Porsche or otherwise) - adds about 7% bhp for about Pounds12k+ and if you go outside of Porsche it can invalidate warrantee. X51 only available for cars made after Aug 05. So I couldn't go the Porsche route as my car was made July 05.
2) Add supercharger (Ruf etc) - adds about 25% bhp for about Pounds14k but invalidates warrantee and drastically shortens the life of your engine.
3) Trade in for GT3/Turbo/GT2 - lose Pounds15k+ on trade-in depreciation, then you have to pay for the jump in model.

So #3 is definitely the most expensive but safest option.