Adding an ARAM only Magic Pen percentage item?

Anyone else think it might be useful? There have been so many games where I end up with four or five AP on one team (given champions aren't chosen), and there is nothing that can be done against tanks even when every pen item. Especially given that tanks now have snowball, so poking them isn't really possible, and champions start with MR. I often see champions with over 300MR. And basically every game multiple people have over 150, even ADC's.

If you end up with five AD vs tanks, you can buy Black Cleaver + LW, but the only percentage based item mages have is void. I am not very knowledgeable of stats and the formula's for damage but even with void staff isn't 300 MR around 65% magic damage resistance? So you are dealing around 35% damage, and many MR items and tanky champions have a lot of health as well as a way to heal so it is barely hurting them. SV is also extremely popular just for the heal given can't go back to base to heal, and the aura from Aegis much more effective as always in groups.

Then they get a large shield from maw if they have it, or increased healing from sv, or the shield from orb of winter (because ARAM has an extra MR item), or from BV.

Yes there is liandry's for health but for team fights I believe it doesn't stack from different members, and is reduced by MR also.

Even if every pen item is bought if they have around 300 MR

300*65%=195 -20 from Abyssal -15 from boots -15 from Haunting =145

Which would leave you still only doing 40% damage, and having low AP/ other stats due to stacking pen. Though I suppose that doesn't take into account Masteries and runes...

Anyway not sure of the math, but thought maybe some others might find it frustrating. I know they are working on a mage update and that ARAM isn't a priority, or meant to be perfectly balance, but as there are some ARAM only items (such as +70 MR from orb of winter), thought it may be something others would like also.