Please note that NW.js is only available with a license and any discussion of ways to circumvent the license restriction is not allowed, even if it is possible.
I cannot think of any technical reason Electron would perform better or worse than NW.js - they are based on the same Chromium browser engine, so are running pretty much the same code. The screenshots of performance results look close enough to be within the margin of error, or deriving from window size differences or the Chromium version used. Realistically I don't think anything would be much different if we supported Electron, and it could be a lot of work.
NW.js is regularly updated and AFAIK is robust enough for several games to have been published to Steam with it. As far as I am aware the new versions tend to work better than the old versions, not worse, especially since they come with the feature, performance and memory improvements of the newer Chromium versions. If you run in to problems please do file bugs for any issues you have so we are aware of them.
The "relying on browsers" argument has been done to death a bunch of times already and I'm really sick of it - you can't avoid relying on third parties in software. If we went native then we would be hosed by crappy graphics drivers instead, and there wouldn't be anything we could do about it. So I know it's frustrating when there are problems caused by other software projects, but there is no reason to think that changing which software we depend on would fundamentally resolve that.