My RSi has a S/S exhaust, de-cat, custom hill power I/K and has been chipped... had it RR'd at hill power a few weeks ago and it only made 111.6 BHP at the flywheel which is around standard power!
If your looking into headwork, fast road cam, and a remap this has made some RSi's upto about 135bhp, but youll be looking at spending atleast £1200, it will be far more cost effective to stick a F7R engine in (2.0L 16V)
As for the vovlo turbo conversion has been talked about a few times, and from what ive read it isnt really worth it, you will need a custom loom made to match clio - volvo parts, a custom down pipe made, and then your still only making just over standard power compared to the RSi.