https://www.youtube.com/watch?v=qC46PDepyUA
I think this replays shows that people exaggerate how powerful the JLI really are as long as they are not in a massive blob. Von's 1-2 squads trade well but are forced off frequently and are not winning the game for him by any means. When there are just 1-2 JLI squads operating seperately they are not that hard to deal with.
Which leads me to believe that for now their combat performance is fine and that they just need a small cost increase alongside a cooldown increase to make them trade a bit less and prevent them from being spammed in blobs.
That's a 60 minute video. Looking at the unit composition, it's up against an Assault Engineer and .50 cal composition which later deploys the Bulldozer: literally a doctrinal counter build.
JLI as is beat the crap out of dual BAR Riflemen in a range 20 green cover fight and have the upper hand against LMG Paratroopers under the same conditions. They're completely off the curve.