Katyusha is an artillery piece, Puma is a Armored vehicle. They have two different jobs so they can't be compared. However, with the OKW getting a Stuka can severly delay you in teching because it's 100 fuel + the OKW fuel income decrease and you're still nowhere near access to any tanks which means OKW don't have much to defend their Stukas compared to what the Soviets might have which is why I brought this up.
Even more reason why stuka should get HP nerf.
Also, the Stuka costs more than the Soviet T34s while the Puma costs roughly the same as the Katyushas. The Puma's gun also takes two hits to kill a Katyusha.
Even if we include fuel penalty, getting stuka is still almost twice as cheap as getting T34.
Getting Katiusha is ~1.7(CBA to get real value, calculate it yourself, its simple math and I have too high fever to do it) more expensive then getting stuka.
And lets go with the survivability further, it takes 4 Su-76 shots to kill stuka. Even more imbalanced now compared to puma.
There is not a single valid reason why stuka should keep 320 hp. Its way too much, destroying it is way too hard. All other arty when caught by surprise is DEAD, stuka backs up while lolling.