Increasing development makes sense to me, and maybe without underexposing. Underexposure will dump shadow detail, which, if the subject (and lighting) are flat (low contrast), may not be required to fit everything into the range capability of the film/developer combo. Just adding to development time will also increase separation between adjacent tones too (like in the middle of the range, from zone 4-7), and with VC paper, you can use one exposure at a low grade to bring in highlights, then a high filter to blacken the shadows below the desired detail level.
My feeling is always fill up the negative. It may take longer to print, but the results are usually richer.