Well, I finally had a bit of a breakthrough. I swapped the medium glowplug I was using with a hot glowplug from my dedicated winter nitro truck, and suddenly my RS4 gained 3mph in repeated testing. That's roughtly equivalent to 2000rpm, according to this car's drive ratio and tire size. As if to prove the increase in speed wasn't caused by removing drag in the drivetrain, it also idled higher, so I was able to close the idle gap a little bit. Enriching the fuel mixture 1/24t dropped the top speed to 47mph; leaning it 2/24t did not increase the top speed, only the operating temperature.

What do RCTech's nitro gurus think should be my next step, based on this info?