I find the idea to re-implement the gun upgrade again good. Otherwise the 222 is fine.
This could easily make the 222 worthless again, as you need more than one for them to be effective.
If you have to sink another 110 mun into them, nobody will build them.
What could work however is bringing back the upgrade, but buffing the standard MGs AI and turn the 2cm into pure light AT, allowing for more flexibility.
Or let it cap while unupgraded, or offer another upgrade similar to vCoH's PE scoutcar that turns it into a mobile cache.