so if i had one CLE-215+ on a rig with 6 1080 TI's...what would the net performance gain for that entire rig? Or will one CLE-215+ only apply to 1-2 gpu's?
It will work with all. The bandwidth requirement is a lot less when you spread to many cards like that, so you might even get good results with 1x risers. You'd want the cards running using PCI-e v3 for best results, though.
A single 215+ is basically going to give you 30MH - 60MH depending on algo regardless of the number of cards.