Can you post a full command line for ethminer??
I don't have any custom config in the commandline except --cuda-block-size 128 (which again is default iirc)
In section 5 of nvidia profile inspector
cuda force p2 state : off
power management mode : prefer max performance.
tried at +1000 mem does ~42.5 on phoenix and ~41.7 on ethminer
https://imgur.com/a/omD0M8z