I too produce a fair amount of B/W work using Silver Efex Pro 2 with my M9s. I’ve been extremely happy with the results as have countless others who see the work.
However, the M Monochrom just does this much, much better. The tonal range and fine rendering just gives you a much better file. Whatever one decides to do to the file during processing, produces better results (naturally we are only talking B/W!) as the original file is so much richer tonally.

The difference between both cameras is noticeable if one’s used to them or if a viewer was to look at images side by side. It’s a specific B/W tool and as such performs brilliantly.

As fas as the low ISOs, generally the Leica M range has always been known as the low light camera. As a result, the M8 was under criticism for not producing good results above 640 ISO. I would personally more welcome the higher ISO settings than lower ones, but at least a quality ND filter allows to shoot with a wide aperture in bright light. There would be no fix the other way around.

Lastly, the Leica look is also about technique – one has to know what one’s doing – just using an old, softer, glowing lens without the correct lighting will just make soft pictures. As you correctly pointed out though, for those who know what they are doing, these lenses exist and work on the new cameras, so it’s great to have the choice. As you know, apart from a small number, most lenses from the 1950s will work with the new digital cameras.

The majority of photographers though, I feel, are after sharp and well rendering lenses (which still have the amazing tonal range and contrast abilities, married to rendering pin point light sources correctly and creamy out of focus rendering). As a photographer I think it’s great to have this choice and to craft images as we see fit.

My current outfit, apart from two lenses, is made up of current lens lineup and I’m absolutely delighted with the image quality I’m producing; not technically (which is spot on), but aesthetically – the images have a feel to them. For me, that’s just perfect

I also don’t care for the high ASAs of the MM because I like to shoot with wide apertures. In fact I will hate not having an 80ASA and also hate using ND filters. Why have Leica abandoned lower ASAs and don’t they realise that not everyone wants to shoot under candle light.

My fear is that Leica are endlessly chasing the holy grail of startling sharpness with the MM and new ASPH lenses. The ‘Leica look’ is vanishing and their pictures are becoming synthetically Disney-Land. I for one combat this by using old lenses, which tend to produce the warmth and fulsom feel of the Leica masters.