In regards to the premium fuel trick,
I really don't understand the logic with this. Condensation in the tank freezing is the most common issue which causes fuel related issues. Premium (high octane fuels purpose is to not prematurely ignite when the engine is running at high temperatures. The best way to avoid condensation is to keep the fuel tank full to keep out the air.
My dealer (Brampton Subaru) did a service record check and did not see any reported issues relating hard starts in cold weather. So I think it is best we report the issue to the home dealership and make sure it is recorded.
Suggestion from dealer:
One thing that may help is that when starting the car is when turning the key leave it in the on position for 4-5 seconds before going to the start position, this allows the engine throttle control servo to cycle improving cold start. We may have to preform an "IDLE LEARN" which teaches the engine computer how much to open the throttle on start up.