While this idea sounds good and cold bring more "realism" in game. But there is hidden "poison" inside - snares. Now, snares cripple engine only if you penetrate enemy armor. If deflection damage will be bring for all AT and tanks, it make any armor pushes more risky action than before. You don't need to penetrate armor, just few shots to cripple engine. And i don't mention about balance of handheld AT, now only PTRS could deal deflection damage, if shrecks, piats and zooks will do it, it will also lead us to rebalance of handheld AT too.
And i think shrecks and zooks blobs become more often event than before. You will get "constant" AT damage.
Not everything would necessitate deflection damage, just proper hard counters, like the ones that are currently ignoring armour. I don't by any means want mediums and lights to have it, they can flank. But those than can't flank shouldn't be a dice roll (or lack of in the current case) to get through armour.
Think of it as mid way between vcoh and coh2.
Also all hand AT already deals deflection damage.