Yeah, as a Tahm player, you should never be close enough to me, long enough to where I can auto you to max stacks, Q you and eat you. That's 3 autos, plus Q plus W. Levels 2-7 that's like 5 seconds of being in melee with Tahm that he needs to be able to get full stacks on you and then stun and eat you. If he is getting stacks faster, that's because he spit W at you or Q'd early, which you should realize is his reliable stun on you, which is when you should then go in and fight him. You don't run around scared of a stun incoming when the stun ability is on a 10-20 second CD. Poke him out, kite out his autos, take fights after he throws W or Q at you. Tahm's unpowered at the moment, if you're dying to one that isn't something absolutely ridiculous like ROA, Nashor's Tooth build, you're playing the match up incorrectly.
As an edited, extension to this comment, other than Thresh who disarms himself with his Q hook, there isn't a single champion in the game that I can think of that punishes the player that severely, for hitting a skill shot. Tahm's biggest counter is he needs to get in close to put stacks on his enemy. The change you have suggested would make him the worst champion in the game, flat out by a country mile. He would be the Magikarp of League of Legends and as it is now, with his current strength, he isn't that far off anyways.