While a low mass, adjustable gb setup can work, I agree that if you're not in competition it's likely not worth the additional cost.
That said, I'm planning a Grendel upper build for plinking and maybe hunting, and might just use my low mass carrier with a new bolt. Then of course I would get an adjustable gb, which would be the only added cost. Since I'm thinking a budget setup, a Wojtek agb is almost the same price as fixed.
The carbine spring in a rifle length receiver extension is a no go. Just spend $15 and get a rifle length spring.