Well there's several aspects to consider.
First of all, VW have cleverly positioned the GTI so that it is not too close to the R. Now, with the Mk6 generation VW already had an engine lined up in the form of the S3 engine. It made sense therefore to only incrementally increase the GTI so that it sat comfortably under the 265 output of the R.
For the Mk7 generation, from what I've gathered VW will be introducing a new engine into the R model which will allow for a significant increase in power. From this has stemmed the rumours of 300+ bhp. If this is the case then VW will have a bit more room under the R model to give the GTI power which is at least competitive.
Secondly, the GTI's rivals are now all comfortably over the 250hp bracket. The Focus ST has 250, the Astra has 280, the Megane has 250, the MPS has 265 and there is also the forthcoming Mercedes hot-hatch to consider. Of course, from the VAG line up it will be the Cupra which is given the power to go up against the heavyweights, but even still, I don't foresee VW further distancing the GTI (negatively) from it's competitors.