If the VH is down, I don't think a KT mission (bound to be very small regardless of if you loose perks from air attack), would be much more dangerous than a mission with three times as many M4's.
Mk 84, so, whats your point? They don't spawn up and attack enemy bases with high perk tanks, and thats the topic of this entire thread; ways to encourage offensive use of tanks.
As to the FlaK 88, I'd say make it free for the first 2 tours, and see how it affects the game (Remember it can be destroyed by a couple 7.7mm bullets if they hit the gun crew). Based on the K/D and usage, the perk price would be set, or it could stay free. I say two tours because the first one is going to have the numbers skewed by people using it because of the new. Once the "new" wears off, its use will drop to what we can expect to see without a perk price.