7950/7970/280x are almost certainly performing better than 380x as they have a 384bit memory bus as opposed to the 380x 256bit memory bus, and this gives them more raw memory bandwidth.

There is probably nothing claymore can do about this "issue". 380x just don't lend themselves very well to mining ZCash it seems.

For similar reasons, my 290xs are all faster than my RX480s. 512bit memory bus vs 256bit memory bus. No amount of overclocking the RX480 memory bus makes up for the lack of memory bus width.