Forget O-320?
Yes, Dan has been very vocal about the amazing fuel burns he gets on his big engine, while running aggressively lean of peak on very long cross-countries. If that sounds like your kind of flying, then go for it. But just imagine how much lower the fuel burn would have been using the same procedures on a smaller engine. The laws of physics still apply -- 200 hp takes more gas than 160 hp.
On the other hand, if most of your flying is touch-and-go's or local fly-in lunches, where you're either full-throttle a lot or it's just not worth the hassle of LOP, then the O-320 still makes a lot of sense. Especially if you're considering a carb. Especially with fuel over $5/gallon.
My O-320-powered RV-6A with C/S prop used to get around 7 GPH doing touch-and-goes in the pattern, which, honestly, was probably where most of my flying was done. The same thing in the Piper Cherokee 140 I had before (with the same engine) used to cost at least 10 GPH. The difference is that at 2300 fpm, it takes less than 30 seconds of full-power climb to get to pattern altitude, and then with an RV you're nearly idling the rest of the time, to avoid running over the spam-cans. A C/S prop makes a much greater impact on climb performance than horsepower does. The difference between a C/S 160 hp and a C/S 200 hp is probably about 700 fpm, but full-power climb at 200 hp is probably over 20 GPH.
With the local fly-in scenario, you're probably flying with your buddies, so naturally you're balls-out all the way. And you're not going to mess around with LOP when you're yacking on the radio and flying a loose formation for a 20 minute hop. So with 200 hp, you're probably burning 10 GPH impressing your buddies while they're burning just as much (at 90% power) to keep up. The guy in your formation with the O-320 will probably say screw it because he can't keep up with your 200 horses anyway and he'll pull the power back to 50% to save the gas. And he'll still land within 5 minutes of the rest of the group. But he'll have spent half the money to get there.
Don't get me wrong -- I'm just as impressed as the next guy with raw horsepower. But don't buy a big engine because someone told you it won't cost any more to feed. If you're worrying about fuel burn you're probably a guy like me who has to consider how much fun you can afford.
The worst of all worlds is to build a plane that you can't afford to fly.