DougC wrote:
I have been wanting to try some silly experiments
using a digital TV dish to make a super-hearing type thing

A parabolic microphone (audio)?
The sintered finish on many of those
makes me wonder how good they will be for that
--without a layer of smoothed epoxy.

[...]finally got ahold of one from someone's trash.
[...]the dish appears rather shallow and has a round profile,
not a parabola at all.

Your vision of how a parabola can look seems limited.
http://www.google.com/images?q=conic-sections
One can look like a comet or a gentle bow in a piece of sheet metal.
Of course, to get a close-by focal point,
it will have a pretty beachball-like curvature.

Maybe you can get a better answer in a more apt group:
http://groups.google.com/groups/sear...-sex&scoring=d

Also just for trivia purposes I am curious
as to what the typical dB gain is for this sort of dish (when used for RF),

This group is a rather broadly-oriented one to post that question.
It also has very low traffic levels.
It's not hard to find a more narrowly-focused group.
http://groups.google.com/groups/sear...sh+gain+-DougC
....and there's always Google Web Search.
http://www.google.com/search?q=dish+gain&num=100

and if that equates at all to sound waves...

That's an interesting question.
If you find an answer elsewhere, be sure to post it here as well.