The grey health/shield mechanics break something in the game that better should not be broken. He is not an assassin, thus if you play properly he should not be able to get to your back line without investing something (like his ult, Flash, and item active or something). However, due to grey health, he can just walk by and do whatever he wants:
In my last game (it was an ARAM, but that is the same as constant teamfights), we faced a Tahm Kench that build a lot of health and tank stats. Later on, in every teamfight he would just waddle through all of us with his 4k health, then start attacking the backline with the super long range Q which slows so much that it guarantees him to get to three stacks even though he is melee, then he devours the backline and spits it out back in front. That sounds like it takes an awful lot of time, and it does, but even if he gets bursted 3.5k upon going in (which is in fact about 7k worth of damage, since he is tanky), he'd just activate E and get an extra 3.5k shield for up to 6 seconds. That is a little too much shield for a little too long time, in my opinion. Compared to the Barrier Summoner Spell(!) that gives a lousy 455 shield for only 2 seconds, 3.5k shield for 6 seconds is a bit hefty.
Don't get me wrong, I think Tahm Kench is quite okay as he is regarding his skill set and I don't think he is "OP". But that grey health concept -- effectively meaning he gets almost two times the life he amounts if his E is not on CD -- is a bit ridiculous. I'd suggest he stores maybe 50% of the lost life as grey health, that is a reasonable shield. I could also live with the amount of health restored from grey health is tuned up by 10% per level in exchange.
But it should be worth a shot to damage him, and not just mean he gets a stronger shield anyway to get plenty of time to walk away again.