Subarus are a great platform for a rooftop tent. Years ago did some research on mounting and a little aerodynamics. Granted, this info is over ten years old but may still have some benefit for you.
Bars - We used to go with Subaru's aftermarket bars, they were rebadged Yakima bars. Yakima had virtually the same bars but were only rated at 110 Lbs (dynamic load) while if you went to Subaru they rated the bars at 165 Lbs (dynamic load) so just for insurance, we would buy the Subaru labeled aftermarket ones. A general rule of thumb, I found Thule worked closely with European marques where Yakima played well with Japanese companies. Sometimes it is wise to listen to the vehicle manufacturer. For example, Nissan Xterra racks were only rated at 115 Lbs by Nissan, so both Yakima and Thule were both carried a 115 Lb ratings too. When I talked to one of the design engineers for JAC, who makes the rack, he was surprised, their racks are really strong. So, we took a look at an Xterra and he was surprised because Nissan uses tapping screws to mount the JAC rack to the Xterra roof. Hence, great rack, great truck, bad interface, means the rack is beefy for show only. Remount properly and it would carry a much higher strength rating. Lesson, listen to the vehicle manufacturer. Oh, and never trust the silly little bars that come with a vehicle's rack, generally most are for looks only and don't carry a strength rating.
Aerodynamics - There is a lot you can do here to help with gas mileage and even handling. A well designed roof tent will act as a wing and slice easily through the wind. This never used to be an issue as old Land Rovers, Cruisers, and Jeeps were aerodynamic bricks and 60 mph was pushing it so aerodynamics wasn't a big issue. Older roof tent designs reflected that. Nowadays with modern SUVs it is really important to look at aerodynamics. If you keep your roof tent on full time, the savings will be huge over a lifetime. Some thoughts to look for when equipping a Subaru with a roof tent are keep the size, weight and width to a minimum. Here is an example.
This is a small Airland on an older Outback. I personally like this installation. The rack is a Subaru branded aftermarket rack rated at 165 Lbs, and the Airland tent is 110 - 115 range. Yes, you could go carbon fiber and then it would be something like 80 Lbs, even better. Width is a big concern, notice how large the tent looks, it is a small tent, only 50 inches wide or so, just a two man tent. If you go wider, like a standard Roost tent, gas mileage will suffer as you get a weird end-plate effect. The tent is wider than the vehicle's roof so it has air coming from a couple direction and is inefficient, you want a smooth flow under and over the tent, remember it is like a wing
I don't have an wind tunnel but you can experiment on your own. Tape small pieces of yarn or cassette tape (telltales) to the tent and to the roof of the vehicle. Have a friend drive the vehicle at speed and watch what happens from a pursuit car. If the telltales are flat or minimally moving, that is good airflow, flapping about wildly is an indication of a stall or drag condition, bad ... bad. Anyway, that is the idea.
Fabric tents - The only thing you can do here is to go with a small frontal area and have a very tight fitting cover. There isn't much you can do with aerodynamics here, they are soft bricks, bad ..... bad. A couple thought for improvement would be to go with a low frontal area tent like an OverCamp, long and thin. Also if you can get the cover to fit tight and not flap, that is a big improvement. I don't know what it is with flapping but on a sailboat sail (wing) it adds a lot of drag and kills power. One thought that I've always wanted to test is to try a fairing, not the silly ones that you can buy from Thule or Yakima that sit under the tent but one for the front of the hood. I noticed that if you add one of those silly bug deflectors like you see on pick em up trucks they keep the bugs from spattering on the tents. They change the airflow over the vehicle but I've never tested it personally. I've heard anecdotally that it helps, well it may save a few bugs lives.
Let me know if this helps
Dr Science