Just did a quick test: Spean Bridge (near Fort William) to Glasgow center.
Michelin: 3:00
RAC: 2:54
AA: 3:03
IMHO they're all on the high side if traffic is reasonable. Based on covering this route twice, once in each direction, I'd say 2:45 maximum barring tie-ups. (I tend to drive conservatively on the left and had a low-powered, though not underpowered, car.)
The AA directions were by far the most complete. The others said, in effect, "take the A82 for 100 miles" with other roads as needed at both ends. AA gave every turn on the A82 as well.