I don't see much FPS increase using Avisynth MT with Avisynth Proxy. The CPU usage seems capped at 25% for the avsproxy executable. Is it limited to single threads?
I just did a comparison with MeGui which gets up to 7fps doing a 1600x900 encode with QTGMC while AVSProxy/Avidemux only gets up to 4.8. I assume the single threading capacity of avsproxy is negating any advantage of using AviSynthMT.