Criels I am trying to do some performance tuning on my computer at the moment. Not sure if you ever found a fix.

I have 3 game instances all running at 2560x1440 on the computer in my sig.
I know that EQ2 loves single fast cores so I turned off Hyper Threading and designated 2 cores per game instance. This didn't seem to help with FPS much. I can run at High settings with all shadows disabled and get 30-40 FPS in the old lands and Faydwer so far.

I tried setting the resolution lower (1920x1080) but the FPS stays the same. This points to it not being a graphics card issue and my two cards don't go over 40% usage when playing.

My CPU and cores all hover around 70-80% the whole time while playing. It is strange that your CPU remains mostly idle.
I am sure you have tried turning off shadows but that was the biggest performance boost for me. It is a real shame as shadows add so much immersion to the game.