Turbos Prefer Premium
Did some regular vs. premium tests after I drove my 2013 Sonata turbo off the lot. Got about 10% better MPG with premium.
Similar experience in my previous car, SAAB 9-5 wagon with turbo.
When my budget was tighter, rule of thumb was to add 10% to regular gas price and compare to premium. If premium was less than 110% of regular, more cost-effective to go with premium.
Turbo and wastegate provide variable compression. If the fuel octane is higher, turbo boost can be higher, effectively raising compression ratio. This can provide more power or higher efficiency - your choice. Non-turbo/compressor engines don't have as much adjustment range, thus are unlikely to get as much benefit from premium.
Tend to cruise between 80 & 85 mph. Get 31 - 34 MPG with premium fuel, Econ mode and a light foot. Light traffic and staying on cruise control gets a few more MPG. Also have the trunk lid spoiler for some aero benefit.
Do not measure your MPG after changing fuel grade or brand until you're at least halfway through the tank. The ECU needs time to learn how much boost pressure and timing adjustment is possible before engine knock starts.