The Yokohama Geolander G015 has been a popular favourite amongst previous generation Subaru Outback owners, before the Wildpeaks were available on the market. Practically, I think both are going to be plenty for off pavement excursions and won't be the limiting factor for our Outbacks. On pavement, reviews suggest the Geolanders are slightly better than the WPs but Im not sure if there are any objective head-to-head comparisons. I have some on order so no first hand experience though.
If you are looking for ultimate backroad grip, then you'll have to look at performance summer tires like Michelin Pilot Sport 4S etc. but I think those sacrifice too many things in other areas (off road, they'd be useless almost, dangerous on ice/snow even) to be worth it on the Outback. I have Continnental Extreme Contact Sports on my other vehicle in the Summer but always dread an early snowfall in the shoulder seasons because the tires become hockey pucks below 0 Celcius.