It still has nowhere near the anti infantry shock value of a T70 or luchs, which can wipe a squad much more quickly and hunt them fown on retreat more effectively.
While people don't buy a stuart to counter a 222, they absolutely do buy it to counter a fht, puma, luchs, ht, or stuka. It is meant to go a lot deeper to fulfill it's roles than a T70 or a luchs, as it's abilities show.
The luchs doesn't need the extra hp to fulfill it's roles, and can make do with micro like the t70 does. It needed it before because of how late it hit, but now it hits with every other light vehicle.
Also, as a side note, the okw has much better supporting at for the luchs than a T70 has, so it's not like the hp is needed to help it survive diving tanks. If the luchs needs 400hp, so does the t70.
You don't have to dive down into enemy territory to knock those vehicles out, 320 hp is sufficient to stay behind your lines fending those units off, but from my experience most players don't rush a Stuart to counter a Luchs or a Flame HT (Zooks & At guns are dealing with that pretty fine) but rather to harass and wipe lone squads or shock your opponent in early-mid game when he has no AT out.
I can also use a Luchs (or a T70 for axis vehicles) to destroy M3, AAHTs, Kattyushas and other low fuel allies vehicles, but people that buy light tanks want to shock infantry that is not prepared for that, the light vehicle counter is just a bonus.
The Stuart was crap at 320 hp and likewise the Luchs will be crap at 320 hp, it means it dies to 2 Captain zook volleys, hence that means Luchs will be completely shut down by a free unit.
The Luchs got indirectly nerfed through the squad spacing changes and its fuel cost increased, while the pen got decreased. It's meant to be a viable unit and I can say that 320 hp means I will be less keen, if not stopping at all to build that unit and stall for a tank.
I don't get why the T70 still has only 320 hp, although it comes latest, but that has nothing to do with the luchs or stuart.