OK, here is the follow-up. I put some 100 octane ethanol-free gas in the tank. After three weeks, the gas came out of the taps clear, so I am relieved that the Red Kote liner holds up to pure gasoline (er, I mean petrol). I was really not looking forward to trying to strip the liner out of the tank without harming my paint job.
While the fuel sat in the tank, I was also getting my carb resleeved. That, of course, is a topic covered in other threads, but I must say, it came back really nicely done. This afternoon, I reassembled the carb and fired up the engine. Wow, what a difference. Even before it was fully warmed up, the engine idled down smoothly and responded well to throttle. The question then is; was the improvement from removing ethanol from the fuel, resleeving the carb, or from using 100 octane fuel? I suspect all three played a factor, but I am primarily relieved that removing the ethanol from the fuel seems to have halted the deterioration of my tank liner.