I've said this a lot recently but Diesels these days are just as tuneable as turbo petrols.
350+ bhp is possible from a 1.9tdi 
- and it will still do 50+mpg. Have your cake and eat it 
If it's mapped properly it should barely smoke too.
Saw a video of a Leon 170tdi with a Milltek exhaust and it sounded great 
Kit here to take the common rail TDI engines up to 340bhp: http://www.darksidedevelopments.co.uk/products/darkside-gtb-turbo-kit-for-2-0-16v-common-rail-engines-2011.html
I can see a lot of people going over to diesel in the next few years. . .
Remember, power sells cars, torque wins races 
And better Brakes win races
There is nothing stopping TDI tuning...except it isn't usually worth the additional development for a manufacturer. By the time you have modified the Diesel you could have built the Petrol for less and I would have thought the MPG saving is negated on all but long runs.
Take the 335d. Stage 1 Edition 30 pace over 1/4 mile? And the MPG in daily driving is probably 6-8 mpg better in typical driving in favour of the 335d. To get there it's needed an extra 1000 cc and an extra turbo plus additional emissions control. But the performance of the car is impressive.
Most of these cars are marketed towards business/company car users so they are unlikely to bring out a mega tuned Diesel although I agree they have tuning potential like any other petrol
if you are prepared to remove the DPF...
Like many on here I have thought about switching a number of times.
