Hi,
my 998 City E was doing 0-60 in a pretty dismal 19 or so seconds when it was standard. Since then, the timing and fuel mixtures have been a bit messed up, and i've fitted a stage 1 kit, at the moment it's running 22 seconds

hopefully that'll drop when it goes in for tuning next week.
a Stage 1 tune for a carb Mini from Minispares will cost in the region of £170, which includes a new inlet manifold and carb needle, maniflow exhaust manifold, lcb downpipe and RC40 box. Oh, and a K&N air filter. Stage 1's are commonly used simply to make the car run smoother, and do have a slight effect on performance figures (Minispares claims a 20% increase in power). Some people even claim improved fuel economy with Stage 1's. Once the kit is fitted, you need to take it to a rolling road session to have the fuel mixture and timing adjusted to suit the new system. It can cost between £70 and £140 for this, depending on how much work is needed.
As for Stage 2 and beyond, i do not have much info. I've seen Stage 2's costing £500+ which is waaay out of my budget, and those kits include modified/uprated heads and camshafts.