Hi neon,
In my own basic testing, and I think this was borne out by some testers, dropping the bucket size to 128 x 128 can be faster for some scenes and pretty much for the reason you mention about keeping more cores working for longer. However going much smaller than that can cause things to get slower.
Your tool would be a good way to test that of course :-). I'm not sure how representative lower quality renders would be of higher quality ones.
Regards,
Jo