I know that the MPG varies between a 4L60E, and a T-56 and the way the driver drives is, but my 4L60E GTO (ls1) got 17-25 city (and thats babying the gas pedal), and I don't remember the highway MPGs.
If you prefer a stick, get the 6 speed, you will most likely be happier with it and I personally haven't hear complaints about having too many gears.
I am building an LS1 Z car, but I am not that far into my project, I am using the automatic transmission, I want to go the T-56 route, but I don't want to add ~$1,500-$1,700 to my build cost. Hope I helped you out some!