how much of a performance loss will i get from having a 8x slot vs a 16x. If i where to have a Asus Maximus vi hero that could only run 2 770's in sli running at 8x would it be worth the extra to buy a dual 16x board or a 16x + 8x boards
The x8 and x 16 refer to the number of pins which is proportional to the number of 'lanes'. Each lane can carry a certain data rate depending on whether its a pcie 1.0, 2.0 or 3.0 configuration.
The graphics cards these days need x16 slots to actually fit into them. pcie 1.0 speeds won't let them run fast enough.