Does your "reminified" CSS actually compress better under GZIP than any of the independent minifiers? The variance you were seeing looks to be irrelevant in comparison.
It doesn't yet, but it could. Keep it mind, at the worst case scenario I get to pick which of the four individual minifiers I run. I will always at least be able to match the best one.
There is absolutely nothing that prevents me from running zopfli over the final outputs and comparing the sizes. I can use the compressed size to drive search. A matter of a change in metric used to consider what is an improvement.
I'll add this to my todo list on this project. Looks like I may be doing more on this than I originally intended...