I suppose as more cars get dyno'd we might see actual reported figures on real cars
Different dynos will give you different figures depending on the dyno and the conditions. You might take your 210bhp car to three dynos on the same day, one give you a figure for crank bhp of say 198, the one down the road 232 and the final one 212...
And none of them are correct, the best use of a dyno is for comparison before and after engine mods...
So relying on dynos to tell you whether the power rating of a GTI PP is over or understated doesn't work in the real world unless you can get yourself a very large sample and make the assumption that the errors generated are random... I suspect the average dyno reads high, for obvious reasons...