1) Turn smart cast off on his ultimate, Devour
2) Abyssal Voyage pings its target location, so if you want to surprise anyone, lock in the location and hit R again to quickly travel there, otherwise everyone will be gone or waiting for you to arrive.
3) For Top lane, Sunfire Aegis would greatly speed up his wave clear if you are going for a tank build. Devour then Regurgitate whatever you ate onto the caster minions then let your Sunfire burn the casters down while you finish off the melee minions.
4) I'm sure there are better setups, but running 9-21-0, Atk Sped, Arm, Scaling MR, and MS quints your first jungle clear is slow but doable starting with one point in Thick Skin.
5) Your passive stacks from An Acquired Taste only show up on champions.
6) I want to say you do a surprising amount of damage even building tank, but I'll defer that judgement until I am up against champions with runes, masteries, and reasonable item buys.

Swain beats Tahm handily once you get a Rod of Ages, before that Thick Skin and your limited mana pool will likely keep you from taking him out unless you happen to be running Ignite.

Tahm Kench makes Nasus cry.

There are some potentially great combos with allies and champions using Devour such as munching your fed Yasuo or Kassadin and running after the enemy team to get your ally into range for say a Force Pulse.