is this modification even worth it? unless completely for fluff reasons would it not be better to just use EX-explosive ammo w/o the HPC?

I think it is worth it, personally, but your milege may vary.

if it were compatible with other kinds of ammo, it would be a must have . .

QFT

So just make it compatible...

Assuming every die averages 1/3rd of a hit, it's mathematically equivalent to APDS and Ex-Ex, excepting 1) cases where either side uses edge, and 2) hardened armor

For case 1, it's advantageous compared to the alternatives when it's the opponent using edge, and worse when it's you, and for hardened armor, it's worse than both.