At the moment subs (particularly T1 subs) suffer from being slow to kill the target (so they can just be ignored by frigates) and it being mass efficient to suicide torpedo bombers into them (meaning even if you have lots of air support right by your subs you end up being punished heavily).
I think it's fine for one of these to be the case, but both of them ends up relegating subs to a very niche unit. If we were talking about some faction specific land unit where there are loads of alternatives that'd be ok, but at the T1 navy stage there are only 2 unit options leading to frigate spam as the favoured option in most games.
A couple of potential simple ideas that might help address this while minimising the wider balance implications (although I may well have overlooked some) - i.e. 1 of the below (not both):
-Double sub torpedo damage, but reduce sub health by 40% (although on the face of it an overall boost, torpedo defence becomes more valuable and means in some matchups subs will be worse off) - the intent for this is to make it harder to ignore subs, without making them more powerful vs things that counter them (so torp bombers will crush them, torpedo launchers will also be a defensive option, and non-UEF destroyers should also do well)
-Double torpedo bomber health and reduce the strike damage by 35% - torpedo bombers wont be able to 1-shot t1 subs, and will need more to 1-shot a cruiser, but should still do well against non-AA dedicated units such as frigates. I.e. the idea is to make it more likely theyll survive to do multiple passes on a unit, while also making them weaker if they're being suicided.