I think I've done a couple of 16 lb'ers on the kettle, but not quite that big.
You just have to worry a little bit more about diffusing the direct heat from scorching the thighs and legs. If you don't already, use some bricks to hold the coals closer to the outside of the kettle The bricks will not only keep the coals from moving in on you, they'll shield that big turkey.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team