Well MEC were considered as "hard" units and were the only ones with the ARM in this category, but in AoD the combined arm bonus is calculated on the divisional level and does not take this aspect in account at all. I'm not sure about Panzer leader trait though.
Is a Panzer leader necessary in order to get the most out of a MEC unit?
Yes, all Mechs get the same +5% for panzer leader as Arm and L.Arm. BTW: Mech are consider Armoured division in the spying menu.
Originally Posted by magic032
Are MECs treated the same as ARM for "combined arms" bonus, when combined with MOTs?
Mechs are threated the same as all divisions. The combined arms bonus is calculated via the average softness of a Corps. The CA Modifier is 0.1 on offensive and 0.15 on defense.
CA-Bonus= CA-Mod x 4 x Softness x (1-Softness). So 50% Softness is perfect, Mech-HeavyArm is the best choice in terms of CA, but everything between 70% and 30% Softness is good.
2 Mot + 1 Arm is the classic combination and the combined arms bonos is very like to be higher than in case of just arm. Lets assume Model 1939 and SP-Art 1940:
[2x87%x(100%-6%)+1x30%x(100%-6%)]/3 = 63.92% Softness => 9.2249% combined arms bonus on offensive
[3x30%x(100%-6%)]/3 = 28.2% Softness => 8.09904% combined arms bonus on offensive
The difference is relativly small. Mech-SpArt and Mech-SpRocketArt are the units with the highest soft attacks after the CA-bonus. Mech-HA can be an alternative but i prefer the highest soft attack.