Doug Williams threw for 16,998 YDS, 100TD, and threw over 1000 more passes than Leftwich in his NFL career. (He also spent 3 years in the USFL)
Byron Leftwich threw for 10,000 YDS and 58TD's in an era where there is more passing done in the NFL.
Byron Leftwich should not be mentioned in the same breath as Doug Williams. Lefty is more similar to Rick Mirer, IMO. http://www.nfl.com/player/rickmirer/2502134/profile