I agree on not modifying the units. I think the one-on-one balance of the infantry squads is in a decent spot at the moment.
What if we did what DBP did to the OST T3-T4 'fork' to USF?
- Lieutenant: 200 MP 50 FU -> 240 MP 30 FU
Captain: 200 MP 60 FU -> 240 MP 30 FU
Major: 240 MP 120 FU -> 200 MP 140 FU
- The AAHT and Stuart now also require either the Weapon Racks or Grenades unlock.
Same teching cost overall on a standard T1 -> T3. Captain's no longer more expensive than Lieutenant: without the old Stuart or free bazooka it hasn't really got a reason to be. Double teching costs more manpower and less fuel.
The AAHT and Stuart lock is to offset huge rush time boost they'd otherwise get. Assuming you tech racks (the cheaper option) the AAHT is 5 FU faster and the Stuart is 15 FU faster.