If you really want only to go faster, then the 2.0 F7R bottom end is the way to go. I almost did it myself, but the deal fell through at the very last moment (breakers actually lost the bottom end!).
The previous owner of my Valver had some work done to the head and cams - and the car definitely pulls better than a standard Valver (ask Smoothed Clio
). Not massive power gains, but then these cars can vary a lot anyway. Mine made about 10bhp more at the wheels than most standardish Valvers did at a recent RR day - although some like BenJs only mildly moded green one was just 1bhp behind. Go fathom! The graphs showed that my torque didnt tail off at 5000rpm like most Valvers though - just carried on at max level to the red line (effect of cams I guess). So the effect is basically to make the car even more prone to surge in the post 4500rpm power band.
But head work is very expensive, especially compared to the 2.0 bottom end. Theres little extra power/torque with the 2.0 - but its spread lower across the rev range, like 60% of max torque at 2500rpm for a Valver and 85% torque at 2500rpm for a 2.0. So a 2.0 will pull a lot better out of junctions etc where you would have to change down a gear (and waste a second) in a Valver.
My advice would also be to stick to standard wheels for best low down acceleration, possibly with smaller radius (and cheaper) 195/50/15 tyres. I went back from 16s and really noticed the difference. Handles better too - less grip, but more feel and adjustability on the limit.