So I've got three 5% suppression bonuses for a Maxim. If I equip all three of these would the Maxim now effectively have the same buff the other HMGs got?
Nope.
The incremental suppression and accuracy got buffed.
It means the more entities are being blobbed, the higher suppression and accuracy will be.
Maxim doesn't have such modifiers, that is why it have a hard time suppressing even 1 squad in the blob walking over say yellow cover.