Question is, can a M lens handle a 40MP sensor. A dealer once told me that Leica have not inched past 24MP in order to preserve their M lenses.
However, in my opinion even the SL is not the perfect solution to use M lenses. Based on my experience the best platform for M lenses remains to be M bodies. SL has different filter stack (I think it is about 1.0-1.1 mm, so bit thicker than the digital M) so depending on the lens a slight performance hit is apparent (e.g. I just hated how my Summicron-M 28 version 1 looked on SL, version 2 = much better). I would use L-mount lenses on SL body (apart from some exceptions, like Summilux-M 75 + SL is wonderful combination).