Surely, if everybody is using barometric altimetry, the separation will be maintained because the errors will be the same.
AIUI, the need for all the extra stuff to do with RVSM (which I am reliably informed can add five or even six figures to the cost of a turboprop) is because normal altimeters have such cr*p repeatibility in such thin air that separation would be compromised.
An air data computer is only a few k.