Does the breast cook faster that way?
The whole idea of trussing is that you get a more consistent doneness on the bird. Rather than the legs and wings hanging out there taking all that heat on their own, trussing draws everything up tight so that it's basically more of a uniform "roast" shape and will cook more evenly.
MY OPINION on why the breast gets done too quick (or quicker than the dark meat) has to do with the direct heat. I've played with this theory a good amount on chickens on various cookers and I think I'm on to something.
In other words, on a kettle, most of your heat (on a typical indirect cooking setup with bricks or some sort of diffuser) comes up and over the top of the kettle, just under the lid. So the breasts are going to get that heat more directly than the thighs and legs. On a UDS, though, since the heat is coming up from underneath, the thighs and legs get it first.
When I do whole chickens (and I do a LOT of em) on my UDS, I will always put the chickens on the grate (usually 3 or 4 at a time) so that the legs are in the center and the breasts are on the outer perimeter of the drum. This way, most of the heat from the fire, as it comes up the center of the drum, is hitting the dark meat first and the breasts, located along the outside, are cooking at a little lower temp than the legs and thighs. Doing this, my chickens usually end up cooking so that the thighs and legs are at 165-170 when the breasts are at 150-155. In fact, if anything, the breasts are usually a little behind, so then at the end, I'll just flip the birds so that the breasts are in the center and facing down so that they catch up at the end.
It would be hard to do this with a turkey due to the size. Still, cooking a turkey on a drum with breast side up will help keep the breasts from overcooking by the time the thighs are done.
PLUS....don't forget to ice the breasts! This will help too.