I guess, at the end of the day, if the original Rover engineers thought they could get more power AND better MPG simply by using a different air filter arrangement then surely they would.
I'm also a Ford owner and this question comes up frequently regarding older Zetec engines. The consensus, by those far more experienced and knowledgeable than i'll ever be is that you need to do a lot to an engines ability to consume fuel before lack of fresh air becomes the restriction.
So, in reality, a well set up V6 on an autobox will not be held back by a lack of fresh air under normal conditions. As suggested, getting the gases out is probably the best place to start before you try to get more O2 in. And then that would probably come after gas-flowing heads followed by remapping and rolling road adjustment.
Each to their own of course but if you want a faster car, buy a faster car (no offence intended). Or try Nitrous