I tend to disagree with the use more fuel theory. I don't think that higher Nitro necessarily means use more fuel. What it does mean is that you use more nitro and more nitro = more power. What uses more fuel is the fact that you are increasing the RPM for each throttle setting by using the same prop. Because fuel draw is more correlated with RPM than with the fuel itself.

In other words, if you could prop it up so that you are getting the same max rpm as you were getting with 15%, you would be getting around the same fuel consumption. Not only that, as Montague mentioned, you would also be getting better wear characteristics.

Also... pulling with authority DOESN'T mean you should up the pitch... it means you should go down on pitch. Lets take 3D planes for example, they are the planes that need to the most pulling "authority" and they use the least pitch. Pitch is like your gears in the car (you drive a stick you'd know better). Higher pitch is higher gear like 5 or 6 on a manual. Lower pitch is like 1 or L. You see, for pulling authority, you use lower pitch but for faster flight you use higher pitch. Then, once you know what you want, you choose the prop diameter to ensure you are in the powerband of the engine.

The contender doesn't look to be a fast airplane and with a thick airfoil like that, it won't need to be going too quick to create good amount of lift. I would recommend you stick with the recommended prop sizes and go UP at diameter and DOWN a pitch if you want more engine authority.