OP,
I just recently went through a similar decision process. The 223 thread has convinced me of its effectiveness with the TMK at 1800fps or above, but I would prefer a round that would keep that 1800fps to a longer range than 400-450. If Tikka offered a 6mm Creedmoor I would have bought that.
I considered the 1:8 twist 243 or 22-250, but I decided against them for two reasons. First, all factory ammo for 243 or 22-250 is intended for the traditional slower twist rifles, so no heavy long high BC bullets factory loaded. So to get any effective range improvement over the 223 I would need to handload 100%. That's not a deal breaker by itself. The other issue is I want to shoot this rifle a LOT. A 22-250 or 243 will be dead in short order when used as a training rifle.
So I ordered the 223. I plan to shoot the absolute snot out of it, including whitetails. Once I have developed the skills to make legitimate long range ethical shots, then I will purchase or assemble a dedicated hunting rifle in 6cm, 6um, 25-284, 22cm, 22um, or some whiz bang long range barrel burner. Once zeroed and verified, this rifle will be used only for shots on animals while I continue to shoot the snot out of the 223.