Anyone know what the size and stall speed of the stock Torque converter in the G8 GT will be?

I'm sure it will be tight since the main reason for having a looser converter (i.e. only four speeds with large gaps between them) is not there.

Any vendors/companies playing with the 6L80/85/90E out there yet?

Not sure, but I would look to Corvette Forum, or somewhere like that as they would have any vendors already dealing with that transmission. I do know that as of now upgraded parts are limited, if there are any at all.