Consider recoil as a major negative. For comp shooting, less is better.
High BC bullets at high velocity is the ideal mix. There are many ways to achieve this goal.
First off, decide what class you want to shoot in if competing. There will be a few winning combos for you to choose from.
If this is just a LR toy, consider if you will put a muzzle brake on or not. How heavy the overal weight of the rifle will be. Shooting off a table/pedestal rest or off a bipod/belly.
This helps with stock choice, which helps with recoil, which narrows down the cartridge/bullet set up. Barrel size and length will decide the type of bedding/action/stock setup you might want to use. Which then leads to balancing overal weight and any restrictions on rifle dimensions.
There really is not one right answer. Just depends on what you are trying to do. Decide on the class/rules, then reverse engineer to the rifle.
At 1000yds/m, there simply is no debate, the larger WM destroys the 308 in any and all ways. The biggest negative is recoil which is substantially higher. But then the class you would want to shoot a 300WM would allow a very heavy rifle, negating recoil issues.
But then there are other, better choices.....and so it goes.
As to Barrels, I too am very fond of Gaillard barrels. I am about to put another to the test in my new 6.5 Mystic 1000m BR rifle.
Jerry