Hi--

>Gee, I never thought about handholding vs tripod to improve sharpness!!

It's all in the damping....;-)

>Does this hold true for, say, a 300 mm Nikkor F/4 + 2X TC as well, or
>is it some peculiarity of a light mirror lens?

It's more true of light, short mirrors (no great Length X Mass to slow motions/vibrations).

>Also, what shutter speed are we talking about for a 600 mm?

At least 1/500th.

>Note- this question is from someone who was never able to get a sharp >picture from a 600 mm Sigma mirror at 1/500 or less using what I thought
>was a pretty sturdy tripod.
>Regards,
>Charlie C. User903809@aol.com

Sturdy does not necessarily imply steady (well-damped) - some of the best-damped tripods do not appear at all sturdy (floppy Huskies and old Vivitars vs. Bogen cross-braced tripods, for instance).
Hope This Helps
David Ruether