Ok. Before you make an offer go to www.truecar.com to make sure you actually know what they might accept.
I have no idea what your scenario is. What I can tell you is, unless a dealer is marking a Subaru up over MSRP, there is no way you are buying a new one at 15% below their price.
Now, there are some brands out there with huge rebates, where 15% below MSRP is more than a dealer needs to get. If they accept your offer; no tricks adding back destination or rebates you do not qualify for, or requiring you purchase something extra (service contrat for example), then you most likely didn't do your homework.
My posts are solely based on my opinions and for my entertainment, contact a professional if you need real advice.
When you get in debt you become a slave. - Andrew Jackson