Main problem is that the fuel difference between StuG and Puma is so minimal that if you are going to tech BP2 then you might as well just stall for the extra 25 fuel to get StuG. Honestly I think the best way to make Puma work would be to make it side tech from Tier 2 like AEC and just cost it appropriately so that going for Puma is a conscious and deliberate decision to counter T70 and the likes.
I used to think the same, but the casemate StuG is awful against T-70s. It's like trying to use a Raketenwerfer against one.
When you're comparing 320 MP 70 FU (I think>) for a Puma to 540 FU 150 FU for a Support Armor Korps and the P4, it holds up more favourably.
Mostly though you use Mobile Defence to skip T3 entirely. Puma, Command P4, Battle Phase 3, Panther.
Decent counter to IS-2 stalls.