Upload & Sell: On
...I'm really craving the added light and smoothness/bokeh of a 50 at 1.4, but the 50lux is not cheap, and requires adapting. ...Would it be so blasphemous to jump ship to the new Sigma 50mm Art (with AF)? I imagine that aside from the weight difference, the images will be sharper than Leica 50s manufactured over 10-20 years ago.
There's nothing the slightest bit wrong with using non-Leica lenses. Reasons to try to the Sigma: probably better resolution and contrast, automatic focus and aperture, warranty coverage, exif data recording. Reasons to go with Leica R: better handling such as manual focus feel and size of the lens, possibly better bokeh and aberration handling, probably better reliability, probably lower initial cost and almost certainly lower long-run cost.
If you like the Summicron mostly because of how it handles, the Summilux is the obvious answer for you. If you like the Summicron for how it draws light, then the only way to find out whether the Sigma will satisfy is to wait until more sample images are available. In the meantime, you could buy a Summilux, try it out, and--if you decide to go another route--sell it for about the same as you paid for it. Win-win.