interpreting gray-scale text as black

I use Ghostscript in HylaFAX to produce faxable TIFF image data.  A 
simplified example of what HylaFAX does with Ghostscript is this...

gs -q -sDEVICE=tiffg4 -dNOPAUSE -dSAFER=true -dBATCH -r204x196 
-sOutputFile=output.tif input.pdf

Occasionally users send faxes which originate as PDFs with color or 
gray-scale text in them.  Naturally, Ghostscript has to produce a 
dithered image result since ITU G4 data is monochrome.

Often the user who sends faxes from PDFs with color or gray-scale text 
will complain that the text is either not readable (dark-enough) or that 
the document takes too long to transmit (due to the increased data 
size).  Ghostscript does an excellent job of rendering blue as a shade 
of gray, but many users seem to prefer if the blue text is translated to 
black.

I would like to implement a way to help those users that does not 
require them to reproduce their source document with black-only text 
(which is what I currently have to do).

I've attached a sample PDF with text in black and then various shades of 
gray.

Using Ghostscript, how would you "shift" the interpretation of the 
gray-scale text such that the output image text would be noticeably 
darker for the gray-scale text or all simply made to be black?

Thanks,

Lee.

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