Sounds like you have an awesome purpose built Forester that is serving you well.
Both tires you mention are solid performers but are built for different terrain. The Michelin Defender is a fantastic highway tire and will hold up to some mild off-roading but it is not built for off-road traction. The Yokohama Geolandar is built as an all-terrain tire but also performs very well on road. I have had a set of them on my FJ Cruiser and now my Dodge 3500 and have been very impressed with them both on and off-road. They ride smooth and quiet on-road and give solid traction off-road I don't believe you will see much if any decrease in mileage unless you change up the sizing.
Many people on the Forester and Outback forums have had good luck with the Yokohama tires.
Have fun (which it sounds like you do) and be safe in your travels. You should share some of your adventures.
Please let us know if you have any additional questions.