cast-xmr will not help with your problem. CN-Heavy doubled it's memory requirements, so now you see only ±50% of your old Cryptonote hashrate. It's not a bug, it's a feature.
feature is good.. but if lost 50% hashrate(hardware that not have 4Mb+).... it's now we mine only 50% from old hashes, -50% profit?
no, because all of us who mining SUMO fighting with the same hash drop. Even Vega cards lost a lot. There is no real disadvantage for you.