An increase in power is completely out of the question. Like fmp said, the development and testing costs would dramatically increase the price. Also bear in mind that they can't make it faster than the Megane. It's a bit like with the Porsche Cayman, they didn't fit an LSD as it would be treading on the toes of the 911 in regards to lap times (shows how the 911 engine/powertrain layout is outdated!).
An R27.r is semi-realistic I feel; strip out the rear, fit harnesses, Bridgestone RE070s, a bright paint/vinyl job and possibly a Quafe LSD. The development costs of this would be a lot lower than a 1bhp increase in power!