That noise seems really high and also the timing seems strange.... it seems like bursts of high frequency noise (similar to the tx frequency)... why should that change to a better situation on full throttle... maybe less pwm from the esc? Anyway the level of that noise seems a lot higher than the transmitter itself... I'm guessing how do those esc really pass emc test... Are you sure the noise is not injected in the transponder antenna? Are you using amb transponders or rchourglass? Maybe have a try with a separate battery and not the bec. I run 1/10 electric buggy and my friends have tested with 1/8 electric buggies, still the performance where not really affected by speed. Of course faster speed means less hits. Our finish line is in the middle of a straight so probably 90% of the times full throttle but under acceleration. Working with real frequency demodulation would improve noise rejection....