You're about dead on with your estimates. I burn 12gph and get an honest 145mph to maybe 150mph in smooth air, leaned out at 10,000'. Skywagons that exceed 150mph regularly, with big tires, no wheel pants or fairings are rare. If we could just get rid of the antenna farms sprouting off these things we'd probably gain a few knots right there.
My airplane has a high'ish time engine in it, but with new ECI cylinders. The bottom has 1300 hrs on it and is tight as a drum. It doesn't seep or leak anywhere. I'm going to fly it until oil consumption or analysis tells me otherwise. My prop is an 88" C203 seaplane prop, which is probably not the fastest prop by far, but pulls like a mule and rattles the windows at the airpark when I takeoff.
I went from an RV-8 that burned 8gph at almost 200mph, so feeding what seems like a tankerload of fuel into the Skywagon was quite startling for me early on, but I've learned to just adapt. For long trips, we'll take a jet tube. For more sporting, flying-as-the-main-reason-we're-going type trips, we'll budget ahead to feed the 'wagon. I just got the autofuel STC so for local flying (95% of my missions anyway), I just throw in ten gallons of car gas for each flight (two, easily managed gas cans), and go tool around at reduced power and have a ball.
If you really want an economical traveling ship, get an RV! For the same money a Skywagon would cost you, you could find a very nice, low-time RV. It won't haul the freight like a Cessna, but if you're mostly flying solo or two-up, it's hard to beat. They're really magic carpets and cover ground in a big way.
Brian