We traded in basically the exact same model/trim/mileage for our ascent. Our dealership matched what carmax offered which is obv less than if i sold it private party but for the hassle of doing it myself it was worth the couple extra grand hit we took.
I'm not sure where your dealership is getting those figures because according to the Subaru GTV chart you (and I) are outside the acceptable year/mileage criteria already to get the quoted amount less the mileage deduction:
The Guaranteed Trade-In Program Value for your 2015 Subaru Outback 2.5i Limited with up to 45,000 miles is:
$19,500^ (Less $0.20/mile over 45k; see below)
This would actually be less that what I ended up being offered on the trade-in. So I did better than the Subaru GTV.
Beginning April 1st 2018, GTP value is based on the following maximum allowable total vehicle mileage at the time of actual sale:
Model Year 2010 100,000 miles
Model Year 2011 100,000 miles
Model Year 2012 90,000 miles
Model Year 2013 75,000 miles
Model Year 2014 60,000 miles
Model Year 2015 45,000 miles
Model Year 2016 30,000 miles
Model Year 2017 22,500 miles
Model Year 2018 15,000 miles
For vehicles exceeding the above maximum mileage limits and not disallowed by the Vehicle Exclusions or the Vehicle Condition Criteria outlined in the Program Guidelines, the GTP Value offered to the customer must be reduced by $0.20 for each mile over the maximum mileage allowance. This is the only allowed reduction of the GTP Value.
Source:
https://www.subaru.com/guaranteed-trade-in-program/rules.html