All Kog'Maw needed before the rework, and what I still think he needs now

Icathian Surprise

Upon taking fatal damage, Kog'Maw remains active for 4 seconds, losing the ability to use abilities and basic attacks but becoming untargetable and invulnerable and gaining bonus movement speed over the duration, increasing to up to to 40% bonus movement speed. He then explodes, dealing 125 - 550 (based on level) true damage to all nearby enemies.

Kog'maw gains additional magic penetration equal to half of his physical penetration, and additional physical penetration equal to half of his magic penetration. This does not stack with itself.

This allows Kog'Maw to build one type of penetration stat to make all of his damage more effective. Prior to the rework, I feel that this is all that would have been needed to put him in an amazing spot. Post-rework I still feel that this would help him greatly, specifically for builds that want to take advantage of his ult's ad scaling to still preserve a poke based play style.

Too strong? Too weak? What do you guys think?