Confirmed, got the same issue on 1070TI: hashrate drops to 17-20 MH/s when mining ETC. Tested on Nicehash and Ethermine ETC pools.
Tried almost all avaiable miner software (PhoenixMiner, Claymore, GMiner, Ethminer 0.18/0.19a) and always got the same result.
At least I'm not alone. That would be very surprising if this was because of the DAG file size difference.