First thing you can't answer "just should have got the m6"!!!!

Every once in while (not often) I like to get a little more sound out of my loudmouth system when accelerating, or a nice "backing down" sound when de-accelerating.
I'll manually shift my auto trans, this will rev higher than in "D" but as long as I never overrev it will this cause any transmission damage?

I've read once it wasn't good for it, but never heard any specifics.

I think the key here is to avoid driveline jerks. The plates in the transmission will take a lot of force, but remember that they are applied by hydraulic pressure. As long as you don't come from neutral with an idling engine which would reduce clutch pressure, there shouldn't be any good reason not to downshift if you want to back down. Having said that, I would make sure you change fluid and filter in the transmission every 20,000 miles or so (more often if using it at the strip). Oil deteriorates from heat, and clutch linings will wear and shed faster if you don't keep the oil fresh. Maintenance is the key.

I would also say -- just try not to over rev and do not do any crazy shifing -- like down to 2nd while doing 100mph...I manual shift sometimes for power and if done in moderation and not too aggressive you should be fine.