Not an expert by any means, but I do fly in 30 to 40 degree temps a lot. Here's some tricks I use:

Take an inverter and hair dryer with you and heat the cylinder and head of the engine just prior to starting.
Run a hotter plug like an A3, maybe even one with an idle bar.
Set the HS mixture a click or two richer than normal and run the engine full throttle for 10 or 15 seconds, then return to proper setting and run full throttle another 10 to 15 sec before trying to take off.
Keep your throttle at 1/2 or more during flight and run it to full throttle now and then, don't try to idle or cruise the whole time, keep the rpms up and the engine is warmer.

I've never tried running 5% nitro fuel, I use 15% omega and it's worked fine for me. I also add a couple onces of Klots per gallon, but I wouldn't say this helps any in the cold, doesn't hurt though as long as your not storing the fuel in freezing temps also.
If you are, it might help to bring the fuel inside the night before you try to fly.

I run O.S 46 and Magnum xls .46 in cold temps all the time. I run 11 x 6 prop on mine also. I know the .51 will spin a bigger prop, but a smaller one will also force the rpms to run a little higher than usual which will help the engine stay warmer and might help U past that midrange problem.

Hope this helps, good luck