Thats just it - if the CS was turbocharged from the factory, then there would be a huge amount of aftermarket parts to upgrade them - you have to remember that engine has been around for 22 years now, where as the F4R has been with us for 11 years, and only dabbled properly into forced induction over the past few years - you cant compare the two.
Also, the reason why we dont all just buy corsa's is speed isnt everything - a 300bhp Corsa will need massive chassis modification - springs, dampers, bushes, top mounts etc etc will add need modifying, where as a 300 bhp trophy could run in standard trim and still be fully drivable.
On top of that, the clio is an awesome handing car, where is the corsa, well, isn't.
You either spend £££ on trying to keep a big bhp Corsa on the road and save a few ££ with tuning, or spend more £££ tuning a Clio and save on the chassis mods - its horses for courses really
Oh, with my itb's I could out pace my mates itb's 2.0 Corsa quite comfortably
Both capable cars, i just prefere the clio