"Low-High" sort divide and conquer with merge... how small to make the subproblems for good efficiency?
My plan of attack is to simply try many different final block sizes such as 1024, 512, 256.... and just time them all. I chose a power of 2 block sizes cuz they can be halved in size easily without any fractions.  For testing purposes, I would force the array sizes to be a perfect power of 2 so they divide perfectly.  My goal is not a generic sorting algorithm but rather more of a divide and conquer optimization speed question.

So in array B, there would be sorted subarrays which I can then merge back into A, so the storage for this algorithm is roughly 2N.

Also regarding the merge part of this (or any sort using merge), why not just start with sorted arrays of size 1 and work your way up?  For example, if we had an array of size 1024, start by merging adjacent pairs of numbers, then merge those pairs into quads...