Experts get NO shield Recovery penalty.
Masters double AC bonus of Shield skill.
Never used GM shield.That is Master 10 shield gets you +20 AC.
(Though Master 10 means just +16 AC over Expert 4.)
You see 100 AC is somehow critical.
After 100 AC Monsters' blows seem to miss a lot more frequently.
And the sooner your percs get 100 AC, the better.
(Y'see, a nice, AC-enchanted shield may bring you extra 35 AC at Expert4, and ab. 55AC at Master 10)
M&M is a Game of Compromise.
Wield a shield. Or a second weapon.