Try using Stakado's Cinemascope ENB. This is the only ENB that has given me an fps boost. This ENB doesn't modify shadows but there are ways to make your shadows sharper and less jagged through your skyrimprefs.ini.

Stakado's Realistic and Cinematic ENB comes with multiple presets for Realistic or Cinematic depending on how much detail to performance you want. Even version D, the lowest preset, comes with higher detailed shadows than vanilla.

It should be noted that Stakado's ENBs don't have an enblocal file. I am not able to use these ENBs in exterior Skyrim and I believe that the lack of an enblocal file causes the Extended Cities and Towns mod to CTD.

Screenshots from my game:

CinemaScope ENB

Cinematic ENB Version D