I use GPS Test Pro (here's a link to the free version
http://goo.gl/lgSPVw) and according to that, its normally within 3 meters/10 feet, and that's with 20+ satellites in view, and using 12-15 for actual location fix. I haven't compared the compass to a true compass, but I have mine set for "True North" not magnetic north. You're right that there is a difference between the 2, and a quick Google search shows the difference is about 15-20 degrees as you said.
As a quick comparison, my previous phones (Galaxy S4, and a Galaxy Nexus before that) showed about the same accuracy, but took slightly longer to obtain a stable location fix.
Edit: I found this article, and it appears that with current smartphones, a 10 foot fix is about as good as it gets for now. Quoting the article "It only takes 5 to 10 seconds of open sky to get a 10-foot fix on the ground, and it doesn’t need Wi-Fi hotspots or cell towers to assist any longer." and here's the link to check it out
Mobile Phone GPS Accuracy? - Tri-State Engineering