You are completely missing the ease of use buffs its getting. Any tank can hull itself down. No support or anything. And they can turn it off rapidly if things get squiffy.
The JP4 would OUTRANGE an, it's already a great TD.
Almost all okw tanks get increased vision with vet, including bonus vision when stationary.
All okw armour has a minimum of 230 armour making them some of the most durable tanks in the game BEFORE adding a damage reduction...
I'm saying there's more to Consider than "hurr durr it'll be cooleo" I know crazy silly things like "faction design" and "will making an already great tank even better upset balance"
Or "what if we combine all things everyone hates about emplacements, make em mobile and let them call in arty"
A hulled down KT with spearhead over watching a VP with a JP4 that could in a few seconds fire at an elefant without risk return fire would be bad design
Especially when you consider that the tank HASN'T given up its mobility-a press of a button and its mobile again, ready to blitz to safety if it needs to...
I'm sayin it's a bad idea shoe honing in abilities that are not considered the repercussions of.
A JP4 could set up shot and outrange a 17lb...
Think man. Think!
I think the problem is you're imagining the OKW hull down being just a copy paste of the Ost one.
While in reality that ability doesn't exist for the OKW, only the animations do, so it's very likely that it's going to be balanced differently.
Plus as I've already said in the Ost topic about it fixed gun vehicles like the StuG, Elefant, Stupa and for the OKW the JP4 should have some sort of cloak or something instead due to their limited gun traverse so it's entirely possible to adjust it to a vehicle specifically altho it already has a cloak ability with vet from what I remember.
As far as the rest of your concerns about OP combination of abilities, you may feel free to test your theory with the All Units mod where you can hulldown OKW vehicles.
I have as well as in the old CoH with the British and it wasn't something that was game breaking again simply because a static position is a great target for artillery.