The fact canvas+ (and really all non-browser engines) lacks so many features far outweighs any performance difference. Web Audio, form controls, XML parsing, standard-compliant inputs, sometimes even letterbox fullscreen, the list goes on and on. Fast and broken is worse than slower and working. If you can delete features to make something faster, it's not particularly useful if you need the features that got deleted. Note that (at least in the past I saw) it has other problems that come in to play as you increase the scale of your game, such as lacking the memory management that browsers have, so games tend to crash due to running out of memory where they would work in a real browser engine.
We deprecated non-browser engines a long time ago and I firmly believe no C2 users should use them, even if they're faster in some cases.
Modern webviews don't have any file size overhead. For example on Android if you target 5.0+, the APK is small compared to the size including Crosswalk. By the end of the year Android 5.0+ should be on the vast majority of devices. It's already at about 2/3rds worldwide.
Ashley I complied for webview on XDK, and for a simple platformer, the performance was far from usable, whereas canvas+ was perfect, speedwise. It completely screwed up alignment of tiled backgrounds and simulate key didn't work, but it was a usable speed, a solid 60 fps, rather than XDK's attempt. Is phonegap better for performance in your opinion?