I would experiment more with the props. Most people don't experiment enough in this area, and often have the wrong prop for the engine/airframe combination. Try dropping down to a 11-5 or a 10.5-6 (both are available from APC and a few others) and allow the engine to turn more rpm to get more into it's power band. Slightly more nitro adds very little to the power in sport engines, but increasing the rpm with a prop change will move most engines to a higher power output.

The Sr. Falcon was originally designed to be powered by .35 to .45 sized engines with much heavier radio gear. Add to that, the engines were much less powerful than today's engines. Of course, way back when, they usually had .45's and larger engines up to .60's that had about the same power that you have now.

On heavy grass, tire size does make a difference in ground roll. I've even seen people put skinny vintage style on for this reason (though it looks strange).