To realize the benefit of a 1:7 barrel you will be loading bullets that will not fit in a normal magazine. All varmint bullets are much lighter than that. 1:9 twist will let you shoot the heaviest varmint type bullets whereas 1:12 will not and 1:7 might show pressure earlier. It is a common twist rate for a reason.

The .243 isn't any harder on barrels than any similar round. It isn't the right round for staking out dogtown but shooting coyotes the barrel will last the rest of your life.