I have a 6 speed T56 in my 2005 GTO. Love it but never use 5th gear. I shift from 4th to 6th on the highway, because I'm already at cruising speed when I'm finished with 4th gear.

The only use for 5th is when cruising at 40-50 mph, and 4th seems just as good or better for that.

My 5th gear is 0.74:1 and 6th is 0.5:1 with a 3.73 rear end. In 6th gear at 65-70 mph, the engine is doing 1,800 rpm. I think the 0.5:1 is just right for highway, considering my rear end ratio. I'm not sure, but I don't think a 5-speed will deliver 0.5:1 in 5th gear, which might be a disadvantage.

I'd say to choose the rpm you want at highway cruising speed and pick either a 5 speed or a 6 speed that will deliver that rpm with your rear end.