Maybe the price cap should track / rise in line with some of the most popular makes / models sold in the open market, in other words if a mid spec Ford Focus, Vauxhall Astra, Nissan Qashqai, Mercedes A-Class rise by 5% from one year to the next then up the cap by the same amount.
The BHP cap is harder to address as its more of a “want” rather then “need” situation – a 115ps base / mid spec Golf will do the same as the 300ps R, it could be argued that given how many Rs seem to be stolen etc very little would be gained from adding them to the scheme. Would go as far as to say that stuff like the R are not really part of the core model range anyway and petrol heads aside most people would dismiss those and other performance / halo models as being to expensive and simply not necessary for their requirements.
Given we are not restricted to base trim / lowest power models and have more or less the same options as the general public would on the the open market the scheme does cater for most if not all needs – as the saying goes “you can please some of the people all the time or all of the people some of the time but not everyone all the time”.