No problems with your gas. The TOC 53 uses a Walbro carb that more than likely was made in the last three years. Those that have contain ethanol resistant components. Don't get caugt up in all the ethanol/methanol bru ha ha. I've been using ethanol blended gas in California for the last 5 years or more and there hasn't been an issue with my engines that normal annual maintenance has not taken care of. So once a year at most buy a $10.00 rebuild kit and replace the diaphragm. You should be doing that with any gas engine if you want to maintain the best peformance.

Boosted:

Your c/g will effect the coupling, along with the amount of rudder deflection. I did not EVER need max rudder deflection for high alpha knife edge. I dialed mine back to about 40 degrees simply because more was not needed. Yes, there was more coupling when higher deflections were used but that's typical with any plane. Just as right to left K/E typivcally requires less coupling than left to right K/E.

Fro a hammerhead there are few, if any, planes that don't also exibit some pitch and/or roll coupling at the top of the turn. Most IMAC and precision flyers take a lot of time and "trimming" flights learning what will be required to mix out various things. They recognize that C/G, engine angles, aircraft weight, adverse yaw from ailerons, and other factors will need to be understood and compensated for in the trimming process.

If you are using the maximum amount of rudder travel that the Sukhoi has available you are inducing a fair portion of the coupling problem through excess. Neither version of the Sukhoi, be it the 35" or the 29%, requires more than 40 degrees of rudder to do anything and everything. With max deflection you are only throwing an aerodynamic brake out into the wind. Try flying it using a little less of everything and see what happens.