I haven't had much chance to play with pos intake vs neg exhaust pressure, the way my box is setup I'm definitely keeping the exhaust suction a bit higher because there's very little downside to pulling in indoor air vs. the negative of pushing outside air into the box and into the house. Something like that will come about in a few months when I have to look at mixing the inside and outside air reliably to feed the miners air above 10°C during the winter and capturing and filtering some amount of exhaust air to feed the house.

The way you're setup I can definitely see the advantage of building a box just to control the air a bit better and cut down the noise a bit,  just remember to measure once and then cut everything, that's the trick the tradies won't tell you