I gave infantry/tanks/vanguards approaching the WWII era +1 to see if it helps the AI
From everything else I've heard, the AI has trouble handling more mobile units. I can't imagine the AI could do better with a host of faster units than an experienced player can.
We already have so many units later game that represent the speed increase of WWII style combat. We have aircraft, naval vessels, tanks, and eventually helicoptors and mech infantry. I don't think the entire core infantry line needs to be made any faster.