You know, it might be a good idea to forget this silly HE round stuff with the M10 and simply reduce M10 fuel cost to 70 so it can once again be a cost effective option for USF. The HE rounds feel rather gimmicky and seem about as appropriate for the unit as the airburst shells were on the Elefant.
I hope that there's no confusion about the functional role of the M10 Tank Destroyer with its unfortunate legacy as a call-in unit and the stale metagames that have persisted as a plague.
If you really want to get wild increase the M10's NEAR penetration from 180 to 200 or even 220(!), but reduce mid range from 160 to 150 and far range from 140 to 120. Alternatively and/or additionally the M10 gun's mid range could be adjusted to calculate at 20 range instead of 25, thus tightening optimal penetration towards closer ranges.
Placing the unit in Major was a nerf indeed, but once unlocked it should certainly be a cost-effective unit against Axis armor. If it's going to be locked in USF's final tier, it must be cost-effective enough to push their opponent towards their final tier. Matching fuel cost with a StuG simply doesn't cut it for a Major tier M10.
What's interesting is that this would actually allow the unit to have better synergy with both Shermans (better penetration) AND Jacksons (more cost-effective for screening).
In my opinion, the best course of action should be to simply ensure every USF commander has access to a tank that can operate effectively in the lategame. Returning the M10 to an effective role would also allow the M10 to be a more suitable option for, say, Mechanized or Rifle company. It'd certainly be more appropriate than the 76mm Sherman's current implementation.
I'd support reducing the M10's cost to 300MP 80FU, removing the HE shell, and see how it performs from there. 70FU rivals the Puma and I'm pretty sure the M10 is more effective than the Puma; it would be far too cost-effective then (You could get 2 M10s for 1 OKW P4).
Deep down though I really think the Jackson and the M10 should also be swapped in this case and that every commander is ensured to have either an EZ8, a Pershing, a Jackson, or SP Artillery (Priest, Calliope.) be accessible.
I'd love to see this, but the issue is that the USF would be doctrine-reliant on dealing with Axis heavy armor. Doctrines with no answer to Axis heavy armor would fail to be of much use (e.g. Recon Support, which has no late-game units/abilities).
Also, if possible, can we sneak an a small buff to the Calliopie?
