In my experience against the Kubel, I think it'd be great if we just left the gun it as-is, but removed the health bonus it got a few patches ago. Also consider reducing the rear-armour to from 2 to 1.5. At Vet 3, it may get it's health and .5 rear armour back, since it deserves it for surviving that long.
Doing that would make the Kubelwagen more vulnerable to flank attacks, and highly vulnerable to AT Guns (one hit would kill it).
I agree with this. It would increase the risk of fighting HMGs head-on and also make it appropriately vulnerable to the M3. An un-countered Kubel would remain as useful as it is now, ie still worth producing.