I don't deny that having UP mainline infantry is a problem, but I don't think it's as bad as having OP mainline infantry. If 1/5 of the factions is 'non-viable', then its simply not played - the other 4/5 continue as normal. If 1/5 of the factions is 'wildly OP', it makes playing 2-3/5 factions incredibly frustrating (or impossible).
Also, I don't think the issue is the RA change (0.9 -> 0.85 out of cover); it works out to something like a 5.9% buff to 'eHP'. The problem is the moving accuracy change (0.35 -> 0.5), which gives IS' 42.8% more DPS when moving - that's what makes them similar to JLI.
/edit
I think a major source of the problem (regarding IS balance) is the massive number of 'states' IS can exist in:
Bolstered?
In Cover?
Weapon upgrades? (LMG, Piat, etc.)
Pyro upgrade? (LOS, Arty, smoke)
Medic upgrade?
Vet level?
Vet 1 and in cover? (more LOS)
Grenades unlocked?
That's just non-doc IS when in combat, without having to deal with auras and abilities. That's an incredible number of combinations to balance. Compared to Grens:
LMG?
Vet level?
Rifle-nade unlocked?
The edit here is exactly why I believe is starting from the start instead of the end make 4 man Tommie work, preferably with the cover mechanic (it's neat and unique) then build from there. If 2 brens on them is then OP, look at that. If bolstered makes them too strong look at that but leave the foundation strong. Part of the reason we are where we are is because bolstered 2 bren Tommie were nuts so brens were nerfed and standard Tommie were nerfed then bolster was delayed and now we're at a state where without any of the "extras" Tommie are not worth their cost or micro.
Start from the stock and work your way up.
Here's to hoping a Brit overhaul happens before the game is abandoned.