+1 to increasing Barrage timer on ISG - that would eliminate a lot of the problem that Brit emplacement have against it and make it more balanced. Just bring it in line with other factions mortars or slightly less.
Comparing ISG and Pack Howie isn't that great at determining if ISG is overperforming IMO... yes they are similar units but are in different factions and used in different contexts.
USF - mobile army dependent on aggressive flanking, who can't set up static screening troops for Pack Howie to set up behind and auto fire endlessly as easy. Also more MP stressed due to Rifleman bleed making getting multiple Pack Howies harder. How often do you see a USF player with 2 Pack Howies?
OKW - Forward Trucks set up natural locations to plop ISGs at. OKW naturally tends to float MP and can more easily build 2-3 ISGs. ISG has longer auto-fire range which synergizes with OKW forward truck screening to fire at troops in a wider radius and not need to reposition.
You see how this can result in the ISG overperforming? Yes 1 vs. 1 Pack Howie can look the same or even better as a unit but the units don't exist in a vacuum. OKW can more easily screen ISGs and build more of them... when you throw in longer auto-fire range... yes the ISG can be too effective when combined with OKW as a whole.
That's because the Pack Howitzer is best used with a Price-style Rear Echelon horde. The cheaper REs, and their surprising durability once they hit vet 2 and 3, allow you to float enough manpower to get 2 Pack Howitzers, provide a meatshield that prevents the Pack Howis from being overrun, and give you a surplus of cheap squads that can recrew them if they do get knocked out.
The Pack Howitzers provide the extra edge in return to deal with an Axis counter-horde of LMG grenadiers or similar elite infantry, as well as MGs.