QTR-Quadtone RIP

"Conventional wisdom" for QTR seems to be to work in grayscale mode, 
but QTR also allows to work in and save in RGB mode. When I convert an 
RGB image to grayscale and apply Levels to set the black and white 
points, the resulting contrast is significantly less than when I stay 
in RGB mode, desaturate and apply Levels to set the black and white 
points. After Convert to Profile, save as tif and print with QTR this 
difference in contrast clearly shows in the prints.

So my question is: why would I ever want to convert to grayscale mode 
given these results, particularly since all my images are in RGB mode 
to begin with. And yes, I can always increase contrast after converting 
to grayscale, but why do this in the first place?

Because storage space is so cheap these days, the argument that the 
resulting grayscale files are smaller is not a very valid one in my 
opinion.

Frans Waterlander