Spatchcocked turkey cooking time
Kenji at Serious Eats recommends 450F for 80 minutes for a 12-14 pound bird "until an instant read thermometer inserted into the deepest part of the breast registers 150°F, and the thighs register at least 165°F". I would expect a 16 pound bird to take about 20 minutes longer (just by figuring a simple ratio).

Serious Eats goes deeply into turkey spatchcocking, so I recommend taking the time to read the articles.

His video here is great.