taking into account that ETH DAG is too big for the cache, I don't believe this rumoured performance (except if this for another algo than ETH). Beside I am not sure that the whole 128 MB space will be available for caching portion of the DAG...
Smart cache holds only data that is in use most of the time, it's a bridge, this is how it works, cpu <--> cache <--> gpu, running a few percentage faster than it normally would be, it does not do miracles but it can do miracles if a type of data is used all the time, need to test and see.