In the standard DXT compression, the Green channel has 1 extra bit compared to the R and B channels, which makes it 565. If you had an alpha to that, I believe it becomes 555+1. I did some tests on this myself and compared a low contrast, high contrast, and a clouds filter as Roughness maps and the extra bit was negligible. Not enough to worry about switching if you or a group of people are new to UE4/PBR and are already comfortable with RMA. Perhaps if you are using really low-res maps the channel switching may be worth it, but I believe I tested this with a 1k on a smooth, black surface and didn’t think it was worth it.