Getting the most out of Avisynth with my new "overkill" machine

not to crash your party, but in my experience related to SSD trim, it does not matter what firmware the disks have, but if the raid controller exposes the availabilty of this option to the os or not. There are some third party tools to work around this though, like wiper: . So you should look into firmware and drivers updates to your raid controller also...

I've basically given up on MT in x64 Avisynth, it always crashes at a seemingly random point. If I need extra performance I just run a few different encodes at once.

You shouldn't disable HT. You can see performance gains of up to 25% (especially in things like x264 which actually thread well).