A Turbo motor can be modified in much the same way as a normally aspirated engine. Adding a modified head, cam or rockers etc will all yield a power increase. The theory is the same, the more air/fuel mix you can cram in there, the more power you'll make. So any way you can help the engine to breathe will help.

First things first.

The first thing to consider is a decent cone type filter from the likes of K&N, Pipercross etc. This is almost essential as trying to fit the Metro air box will be difficult and will take up loads of room.

A decent exhaust is essential. The standard Mini system with it's "Peashooter" size pipe will be next to useless on a Turbo motor. Get a good freeflowing system from  the likes of Maniflow, Playmini etc. If you can afford a stainless one then that's even better. They usually carry a lifetime guarantee.

Cylinder Head.
A well modified cylinder head is perhaps the single most important thing to consider when a power increase is sought.

Need a modified hi-boost head ? Have a look at our range of performance heads here.

One of the areas where the biggest power increase can be seen is the cylinder head. In standard form,the head is very restrictive, 
especialy  on a normally aspirated A-series, but even so when forced induction is concerned.
All the same methods apply, both the inlet and exhaust ports can be opened up, bigger valves added, 3 angle seats and valve guides bullet nosed etc
A well modified head alone will give a suitable power increase, but when coupled to other modifications, such as a cam, exhaust change etc, it will really become worthwhile. Opening up the chambers to lower the compression will allow more boost to be run & this is turn should lead to a power increase.
We can open up your chambers for you, checkout the cylinder head services here.

Camshafts.

There are a huge range of cams available for the A-Series, from the likes of Piper, Kent etc. Everybody has their favourites. But i would suggest not going to "wild" on the choice. Larger cams will decrese low end drivability, and only provide power higher up the rev range if the Turbo is capable of higher revs {maybe ok for a track car} the smaller T2's especially usually run out of puff at around 6,000rpm. So a wild cam just isn't needed. On the road most of the useable power really needs to be below 5,500-6,000 rpm. Try and match your choice of cam to the application it will be used for.
Combined with a good cylinder head, an uprated camshaft will make an excellent power increase.

Hi-Lift roller tip rockers.

Hi-Lift rockers increase valve lift at any given point, and can give a useful power increase. Though depending on cam choice they may be a little over the top for some applications. You may find bottom end power is  decreased slightly in favour of an increase in the mid/high rev range. They are available in both roller and none roller tip. The roller tip having the advantage of reducing side load on the valve stem, which in turn helps to increase valve guide life, & reduce the wear acossiated with high lift applications.
The standard A+ rockers don't even manage a ratio of 1.25-1, so even if the 1.5 ratio items are a bit much for your application there are 1.3-1 available, or to the other extreme 1.7-1. Also available are full roller rockers, which besides having roller tips, also run on needle rollers on the rocker shaft, though these are pricey and may be a bit overkill in most cases.