Cost reduction will "increase" veterancy gain since experience gained from damaging is based on the unit's value, so the latter might end up being unnecessary. The lower the value of the unit, the more experience they can gain from damaging their targets.
Oh yeah and nerf a lot of Allied infantry vet.
That's actually not how it works. Pretty sure it makes them worth less XP when being shot at, but gaining experience is another thing which is separate.
Generally the XP requirements tend to be double the unit's cost and the next level is just the previous amount multiplied by two. Not sure on OKW as they have five levels though the gap I don't really notice with Obers as they consistently hit and kill things with ease.
Responding to Dullahan: Main issue with the OKW vehicles is that their hardcounters are generally on the field and they are risky investments. They can be high-risk-high-reward, especially vs unprepared player, but the most common build orders have their counter. Brits have mines and can get a very quick 6pdr, USF captain tech is a major counter and Soviet T3 does have response units as does T2. Of course they can help win the infantry war, but they can be overwhelmed as well or forced to pull back before they can do enough.
Regardless, the main issue I'm talking about is that the OKW's starting roster tends to crumple without any sort of doctrinal support or Obers to anchor the line on, especially when the WFA/Brits get vet. Obers are good, and are godlike when vetted, but that's resting a whole lot of the army on their shoulders to defeat hostile infantry. They don't have snipers to inflict MP bleed or to ignore those received accuracy bonuses and leigs aren't a replacement for MGs. I wouldn't mind making Ober vet less ridiculous in strength along with the doctrinal support if the doctrinal/Obers weren't completely necessary to compete in the later stages in the game against infantry.