[Les Cargill[_4_]]

Outside of pathological floating point cases ( most likely for filters
with some sort of feedback ), is there ever a good case for using 64 bit
math internal to DAW elements ( read: plugins ) rather than just 32
bit?

I'm not really seeing one. And for Intel processors, 32 bit has
possible performance advantages if you go with SSE math. SSE stuff
(very) roughly uses a 128 bit "register", in which fits four floats,
but only two doubles. So you get a hypothetical-but-not-really
2x speedup just from that for things like long complex multiplies.

Long vectors, being half the size, will also fit in cache better.

I see a breakdown along the lines of "if the plugin uses an FFT, keep
it in floats. If it's structured more like an IIR/FIR filter which
uses less internal data storage, 64 bit may or may not be any better,
depending on how small sample and internal values can be."

--
Les Cargill