Renault clio 1.2 16v straight through exhaust? performance?

You may have big problems if you try to ditch the cat. No doubt the Engine management chip is set up to correctly fuel the engine with the cat in place, changing the exhaust will also change the back-pressure, which in turn will alter the gas flows through the engine. It may be possible to do it, but I'd bet money the EMU would have to be re-programmed to enable the engine to run correctly.