I've been weighting 257 Roberts case weight. The R.P cases that I have weigh from 168.4 to 174.0gn. Nosler cases weigh between 165.8 - 167.4gn.
Is the 5.6gn variance in case weight for R.P cases enough to make a big difference in pressure when loaded?
It is difficult to know, as the difference in weight could be down to differences in the size of the extractor groove cut or whether they have been trimmed to the same length.
It is a pretty small difference too: if both lots of brass have exactly the same size extractor groove, are fireformed, sized and trimmed the same, 5.6 gn of brass would equate to about 0.65 gn of water, or about a 1% difference in capacity.
FWIW though it is a good practice to keep batches of brass separate, and if you are changing brass, or any other variable, to be wary, especially if loads are on the high side.
HTH