The only posiblities for Improved Mech Infantry is one or a combonation of the following:
They become 2/2
They can blitz without a tank
They can carry an infantry with them at 1:1
Personally, I'd hope that it's the middle one. It seems to be the most historically correct IMTO.
Becoming 2/2 implies an increase in lethality which is more like a late Cold War development (Bradleys and other MICV/IFVs) whereas blitzing would be a doctrinal development and enhancing the mobility of your logistics to continue offensive operations.
As for the last one. I can't see the lift capacity increasing... while I'm guilty of thinking of APCs as armoured taxis it's not like a mech inf formation carries a bunch of empty vehicles on it's order of battle just to pick up strays on the side of the road.