I would suggest to at least try and see if it can reduce your rendering times.
There is one "unofficial" modified AviSynth 2.6 build that supports multi-threading and is still being developed. You can also find a short example on when to use which MTMode()-setting. The thread on doom9 gives more insight:
http://forum.doom9.org/showthread.php?t=148782
If you need to use 2.5.8 because of plugins not supporting 2.6, there is also an older version with multi-threading support here.
Maybe it is worth a look.
JEDIT: Thanks, haha ;) Looks like we have gone a far way in this thread.
Found a mirror for the 2.5.8:
https://sites.google.com/site/pavelsx2/ at the bottom ("avisynth258MT")