I took my new flush cut / sawzall blade and chucked it up in my
sawzall. I set the sawzall to a low speed (about 1.5 - 2) and it took
about 30 seconds per jamb.

Interesting idea - glad it worked for you! I would have thought that
the super-flexible flushcut blade would tend to bind/fold/kink as it
gets pushed on the forward stroke of the sawzall. You apparently
didn't have any problem with this? Did you have any orbital action on
the cut?
Thanks for sharing,
Andy