Even doing a fair bit of personal and driving a V8 heavy car i still break even at the very least on fuel. And thats at 21ppm. People jump at fuel cards with little thought to whether they are worth it, its much the same as the situation with the cars themselves.
I have a slightly sensible 3l Diesel, and i generally make a couple of hundred quid a month from the fuel so yes, running something a bit more thirsty is perfectly possible. However, if (like my friend for example) you live in yorkshire, for some reason choose to buy an RX-8 and start seeing a girl in cornwall then the fuel card may be a better option