Turbocharging a normally-aspirated engine can be hard (and expensive!), the biggest limitation on the a2 would be the ECU, as it would probably have an upper limit to the state of tune...hoever, you can get alot more out of a turbo'd engine than a n/a engine.
I come from a bit of background in old mini's, and the standard 1275 engine makes abut 50-60bhp, this can be tuned to 120-140 n/a, and there is a car with aproxx 220bhp, runing a turbo'd 1293cc engine...if you have the money/methods, anything is possible...
The best way to tune the petrol a2 would be to enlarge/port the cylinder head combustion chambers, fit larger inlet and exhaust vavles, gas-flow the inlet/exhaust ports and inlet and exhaust manifolds and fit a free-flowing exhaust system, higher compression ratio if staying non-turbo'd, maybe a slight bore on the cylinders & larger pistons to a bigger capacity (1.8 maybe...?!) and possibly fit different cams. if you could get the electronics sorted, then you could be getting on for 200bhp (for the 1.6fsi), but then you have to consider what the gearbox can handle, and reliabilty becomes a problem.
I would love to find a scrap a2 engine, strip it apart, and see what can be done......
Don't forget, the basic rule of getting more power, it to get more air/fuel into the engine, modern cars are very emmissions orientated (especially the A2!) and they the engines tend to have rather restrive breathing for both fuel & air.
2003 52 Ebony Black A2
1.6 FSI Sport, DIS, Open Sky, 6 CD Symphony II, Auto Climate, rear electric windows, 8 speakers, front fogs.