Simulator running in scaling mode for iPhone 6 Plus?
It looks like setting a launch screen file in the "App Icons and Launch Images" section of your project settings, or adding iOS 8 images to your launch image catalog, will enable "native" resolution.
Adding a correctly sized LaunchImage seems to be enough (setting an incorrectly sized image won't).
Also adding the new "Launch Screen File" should work:
You must set correctly sized LaunchImages
For more information about LaunchImages and icons see this documentation