James, congratulations on the success of your business venture. I'm impressed!
You have 3 choices among 40mm lenses:
Leica 40/2 Summicron-C or its twin the Minolta 40/2 Rokkor, both from the '70s, are king of this range & can be found on the used market.
Rollei 40/2.8 Sonnar is another premium lens in this range, designed by Carl Zeiss & licensed to Rollei. It is available new at B&H for $650.
The new Voigtlander 40/1.4 Nokton is highly desired for its extra speed & compact design. It will be interesting to see how it is received once somene can shoot some pictures with it. For example, there is fairly unanimous agreement that the Leica 50/2 Summicron is a superior lens to the 50/1.4 Summilux for general photography. In order to achieve a maximum aperture of f/1.4, other compromises had to be made by Leica in the design of the Summilux. Similarly, the old Nikkor 50/1.4 rangefinder lens of the '50s was/is a fabulous lens, the favorite among pros of the day. However, it was optimized to be shot wide open & was great at maximum aperture & at f/2 - perfect for low light shooting & for specialized subjects like portraiture. However, this lens was considered to be a dog by f/8.
Different horses for different courses. Depends on your shooting style.
Yes, the increased magnification should make focusing shallow depth of field shots more precise. It would certainly seem that Cosina released the ultra fast 40 Nokton with it to underscore this fact. It would also appear to be the camera of choice for Cosina's other ultra fast lenses like the the 50/1.5 Nokton & the 35/1.2 Nokton - whether you use the latter with an auxiliary finder or estimate with the built-in 40mm framelines. It also has advantages for more precise focusing with other wideangles, which can be tricky to focus & sometimes give a problem to autofocus cameras. Finally, this camera should be able to handle a telephoto faster than the f/3.5 that Cosina currently offers. It wouldn't surprise me at all to see that the next new lens from Cosina is a 90/2.8.
Happy shopping.