Is it safe to smoke a 20lb turkey?
The main concern with smoking large whole turkeys is that heat may not get into the cavity, so the inside may not get up to temperature fast enough. When you spatchcock a turkey the heat can get to both sides evenly, so you take away this problem and a 20lb turkey should be fine. Kamado Joes (I have a Classic 2) have excellent heat circulation so I wouldn't expect you to have a problem with a whole one, but spatchcocking will give you better results.