The highest PPI of any of the grayscale kindles is also 300 ppi.
I haven't looked, but KCC most likely uses one of the common graphic libraries in its conversions.
KCC is open source, you could check what provides the graphic functions:
https://github.com/ciromattia/kcc
From that, you should be able to figure out what the range of the inputs would be 'best' for the conversions.
And the answer is:
It uses the Python Imaging Library:
http://effbot.org/zone/pil-index.htm
Which looks a lot like it is an interface to the imagemagick library.