Funny, i'm in nearly the exact same situation, even live in Marietta, GA.
What I am going to do (have 2 Antminer S9's on order) is to build an enclosure for a 4 post rack frame, cut two large ports in both sides at the bottom where I can install two traditional home air filters to keep the air going into the enclosure free of dirt and debris (since it will be in the garage, not the cleanest place). Additionally, I am going to cut a 12" circle exhaust port on the top, attach ducting to do exactly what you describe, vent up along the garage roof line until I can turn it up into the attic and vent out one of our decorate existing vents on the front facia of our home. Should be relatively easy to run this. To help exhaust, i'm adding an in-line vortex fan to suck the hot air out and force it out the vent. Lastly, to combat the temps you describe, i'm going two things:
1.) Inside the enclosure, rather than just sitting the S9's in their rackmount enclosures (which I also have) and letting the fans vent hot air into the enclosure affecting the surrounding temps, i'm going to install individual tubing from each S9 with a shroud and channel it to this exhaust port.. This way, each S9 channels it's own hot air through an isolated tube to the exhaust port, keeping the surrounding ambient temps inside the enclosure lower..
2.) I'm going with a floor A/C unit, probably from Tripplite, and cutting 1 additional port in the bottom side to pipe in A/C'd air directly into the enclosure.. If this isn't adequate, as bad as I would hate to, I could install a second floor unit with another hole on the opposing side.
This should be adequate to cool them during the summer months, I hope
Thanks,
Matt