It looks like Avisynth 2.6 with MT support is not going to be here anytime soon.. IanB has said that he's not going to go the SetMTMode way so that means that MT code will basically have to be written from scratch. And as Avisynth 2.6.0 hasn't seen an official alpha release in over a year and a half, things are probably not going to process too fast
Too bad since MT-capable CPUs have become very common items.
-Vit-, did you ever notice any improvements regarding stability by using customized plugins you mentioned earlier in this thread (the correct header files)?