I typically do 12-pounders, but according to discussions here, a 20-pounder will fit in the WSM (first concern). A low & slow cook, however, is not recommended-- too much time in the "danger zone" between 40 and 140*. Turkey is best done at 325-350*, and you can achieve this in the WSM with an empty water pan and a full charcoal ring of coals lit standard method.
Brining the bird would be a plus, too. You need to take the internal temp to 160* in the breast (170* in the thigh)-- resting it for 20 minutes before carving will allow residual heat to bring the temp up to 165/175*, and retain juices. My guess would be to allow 4 to 4-1/2 hours to reach these internal temps, cooking at 325-350*-- my 12-pounders are usually done in 3.