It should be easier to add settings to WebCore and WebKit. Currently, there is quite a bit of plumbing you have to do, to surface a setting as an API preference, and it's easy to get wrong.
I'm not sure on the exact balance yet, but I leaning toward a system where to expose a setting via WebKit SPI you need to edit two configuration files (API will require something hand written on top of those edits): the Settings configuration file in WebCore, and Preferences configuration file in WebKit. We could conceivably get away with just the one in WebCore, but that feels like too great a layering violation, and would make it too easy to accidentally expose something as SPI that was only meant for testing.
With 178634, WebCore will be converted to a YAML based configuration for Settings.
I think adding a similar file for WebKit, where you can both declare a WebCore setting that should re-exported via WebKit, and add new WebKit layer only settings.