We have 4 r9 280x are doing mhs 17.88 each and a normal 18.730 ?
This dual MINING eth + dcr
Total 72.833 mh / s
Are your cards 3 GB or 4 GB?
R9 280 and 280X's are all 3GB. eth and etc won't fit but there's other coins that will, like expanse, ubiq, callisto, and so forth (I asked an admin of wattpool to add the DAG size of the many ethash coins on their pool, and he did so you could check what coins are viable there). There were a few 6GB 7970's which the 280X is based on.
the R9 285/380/380X have 4GB models available.