The government will only give grants for certain models of cars. Clios arent included.
To make things worse; I was speaking to a source in the petrolium industry, who said that the government might introduce duty on LPG this year.
Therefore if you spend say £1000 on a conversion, you will take a lot longer to break even.
As for performance, there are different types of kit, suited to different types of vehicles (varying in price). AFAIK, a cheap basic kit might affect performance, so youll need to spend more money to get the same performance as a standard 172.