I don't own the guy and didn't get a chance to test him on PBE, but I'm pretty sure that's supposed to be his identity. Without it, he's waiting on Q, because his basic attacks are only used to prep 3 lobster stacks.
You have to think as a whole what Tahm provides on a consistent basis. There's no point in him treating basic attacks as big dps when, even with his bonus health->damage passive, he isn't threatening enough in fisticuffs.
Think of what any other tank/support does CONSISTENTLY in combat. Leona has a bunch of lockdown on moderately short cooldowns. Naut has more lockdowns and even some damage with w, plus his shield doesn't require him to take damage before being effective. Alistar is old, so his q and w still have longish cooldowns, but he gets immense damage reduction on top of bonus AD to make his attacks hit harder--his heal is meh. Despite not being played much, Taric's passive allows him an opportunity to continually stun, heal and shred enemy armor. Sona has short CD spells, so her power goes without saying. Janna has cc and a shield that lasts for a healthy bit of time, effectively meaning her power is noticed throughout a fight. Soraka has heals for days...
I imagine if you increased the cooldown on Tahm's w, it wouldn't feel like he's eating people enough. That's really what he offers, and I see no reason to remove that from his identity. What else would he be doing if he didn't have a reasonable cooldown on devour? Q doesn't go through other enemy units, and he doesn't have the mobility to guarantee he'll reach his targets. Every time I've played with/against him, he is fairly easy to run away from. If you dodge the Q, he may as well give up the chase.
EDIT: if the damage is too high, then shift the power into better AP scaling so that he gets the damage when he really goes dps. Otherwise, I disagree with increasing the CD of devour.
