Ah I got the wrong end of the stick so apologies for the mixup, I thought you meant that dual gtx980's was too limited and not enough when it sounds more like ENB can't utilise that power past a certain range more like. I wonder if you would get it to manage it if chucked more power or if there is some other issue when the higher you go the more inefficient it would get. By the sound of it is doesn't sound like enb does those things in a linear way and you are hitting some limitation and are at the envelope of its capabilities.
Probably more money than it is worth to try but I'd love to see a stream if you did get it to manage it with everything turned on. Not sure I'd drop for 4 x titans or 4 x 980ti's to try it but it would be sweet to see. I tend to try and aim for 60fps myself, I know skyrim in particular used to have weird physics at varied framerates or over 60 (I have vsync on anyway) so that is another reason to keep it stable which is probably part of your reason I imagine.