This was something that I "played" with when trying to see what kind of usable EI I could get to enable me to use my RB-67 in even the lowest lighting scenarios. While the optics I own for the RB is simply the best available for medium format, they're also very slow. I have the 140 macro lens which I also use as a portrait lens and I usually prefer available light portraits. Delta 3200 gives me that capability along with a reasonable degree of enlargement.
Here's a sample image that I made in my studio of my most favorite mutt !

D6400

 

Click on the image to see the high magnification (336K) scan. This photo was taken with the RB-67 SD hand-held fitted with a 140 Macro KL lens.  Shutter speed was 1/250 @ f/8.0 and I used only the available light in the studio. She is sitting on a sectional sofa that I have about 12 feet from my front window. The film of course is none other than Ilford's Delta 3200 which I pushed to EI6400 in good old Kodak D-23. Grain is fine and sharpness acceptable.


Here's a hi-mag enlargement made on my D-5 using a 75mm lens. The equivalent enlargement would be a 24X30 which even from a 6X7 neg can be a little grainy. However I think that the D3200 has acquitted itself quite well.

D6400

Again, click on the image to see the high magnification (481K) scan

Development details :


D-23 , 1:3 , 35 minutes @ 70F