It's not a massive boost, it is 30% more damage. Only really useful on heavy tanks that now might take 1 less hit to kill.
really? p4: 640hp * 0.7 = 448 theoretical hp with mark target. thats 3 hits instead of 4 with every 160 dmg weapon (shermans, t34s, su85 etc). its 2 shots instead of 3 with 240 dmg (isu, m36)
and thats the problem. while penetration would enable you to score more hits that actually penetrate. dmg increase simply negates a huge potion of the vehicles HP, which imo doesnt belong in a game, where unit preservation is such a big deal.
a simple example: if 3 x 160 dmg vehicles shoot at and penetrate a p4, that is marked, itll die instantly. usually youd have 1 more hit to escape, which gets denied by mark target. no other ability ingame reduces a vehicles theoretical hp like that. this gets worse since crits are tied to hp and hence occur more often on vehicles that have been marked (in this example the 3rd shoot could crit main gun/heavy/abandon instead of the 4th, too)
edit: to mention a few other (actually used) vehicles:
Panther (both factions) 800 -> 560 hp
Tiger 1040 -> 728 hp (3 jackson shots and a scratch on the paint and its gone... instead of 5)'
Luchs puma Stugs 400 -> 280
King/Jagd/Sturm(for the fun of it) 1280 - 896
if you dont see why thats wrong, i cant help it. everyone is aguing "lol if you lose a marked vehicle its cause you didnt react, etc" when waht mark target does is reduce the time you have to react at all.
again: 3 sources with 160dmg cause damage to a p4 that is maked = death
with pen-increase instead of dmg-increase they would have to reload for 1 additional hit and actually give the p4 a chance to react