The health/damage system for vehicles works that way that most of the time you don't have any overkill vs vehicle. For example medium tanks have either 640 hp or 800 hp and most of AT guns/tank weapons deal 160 damage. That means that you need either 4 or 5 shots to destroy a medium tank and you won't do any overkill.
The problem with HEAT shells is that they increase damage by 30% and this doesn't really work with that system. In most cases this actually doesn't decrease the number of hits that you need to destroy a vehicle. For example p4 deals 160 damage normally so you need 4 shots to kill a cromwell. With HEAT shells p4 deals 208 damage. After 3 hits it leaves cromwell at sliver of health but you still need 4th hit to finish it off. So HEAT shells realistically only make the opponent to spend more time repairing his tanks. They really work only in the case if the opponent didn't fully repaired his tank so it doesn't have that sliver of health that could have saved it. In comparison to HEAT shells other damage increasing abilities like mark target or mark vehicle increase damage by 35% and 50% respectively so they don't have this problem.
The solution is to increase damage from HEAT shells from 30% to 35%. I'd say increase munitions cost by 5 too but this ability is already kind of expensive (45 munis) so I won't.