40 pci express lanes is not about performance, it is about additional connectivity without 3rd party chips. X79 allows quad SLI natively while Z87/Z77/etc do not - with 16 lanes you can do x8/x8 dual SLI or you can do tri sli on some boards in x4/x4/x8 configurations. Of course, Tri SLI at x4 speeds will be horrible. You can workaround this with an additional PLX chip for quad SLI, and all of those boards are prohibitively expensive - in the 400$ range (example: Asus Maximus VI extreme or P8Z77 Premium)
"Normal" Z87 boards only support dual SLI in x8/x8 or triple SLI in x4/x4/x8, while X79 allows quad SLI in x8/x8/x8/x8 without the expensive PLX chip. The PLX has a drawback of being very expensive (80$ IIRC just for the chip) and it is slower than native CPU lanes. Examples:
All of these PLX boards are prohibitively expensive, for 400$ you may as well just get a LGA 2011 CPU with a 250$ x79 board. Anyway, long story short - the larger number of lanes on x79/LGA 2011 is there to accommodate more GPUs and more connectivity natively, Z87, Z77, Z68 cannot do this because they only have 16 lanes. Unless you add a PLX chip, which is inherently lower in performance than native PCIE lanes.
As far as performance differences between 2 way sli X16 and dual X8, it is small, probably 4-5%. But the point isn't the performance, it is more GPUs for workstation boards (or gaming if one so chooses) and more peripherals on the PCIE bus. 16 vs 40 lanes is pretty huge in terms of connectivity..
(And no, don't even start typing it: "it doesn't bottleneck" is a childish response I always get about it, not even 16X on a single card is "100%", there is always room for improvement.)
That's the tone of every post i've seen from you. You could easily ask this question without a combative tone, that seems to be the trend from you. Stop trying to start arguments where it isn't needed.
Edited by xoleras - 8/19/13 at 11:42am