290x Crossfire which slots

I have a gigabyte Z97X-Gaming G1 WIFI-BK w/ a plx chip and two 290x (they need to be beside one another - because watercooling config). I figure I have two options:
The first - I put it in the 1_x16 (x8 effective) slot and the other below it in the 1_x8 slot. These two slots share the same 16 lanes through the plx chip.
-or-
The second - I put one in the 1_x8 slot and the other below it in the 2_x16 (x16 effective) slot. These two slots do not share lanes through the plx chip.


The second option seems like the better idea, even if the gains are marginal, but I'm not sure if the second option will result in some sort of increased latency (ex. frame pacing) that outweighs the benefit of using a full x16 slot. I've read up on plx chips a bit and I can't come to a conclusion.

*Side note: I will be using one of the available x16/x8 slots for an x4 card, resulting in 3 of the 4 slots being used. Not sure if this matters much though*

motherboard link: http://www.gigabyte.us/products/product-page.aspx?pid=4980#sp

Which setup? Halp?

You'll want them in the 2 closest to the cpu.

This is my triple setup.

As a product of how my cooling system is arranged they are presently in the 1_x8 2_x16 slots but I will be moving them up to the 1_x16(x8) 1_x8 slots once an adapter comes in. Since I'm moving them I figure I may as well test the difference for posterity, provided I can find a utility to measure this difference.

Does anyone know of a utility or means of measuring frame pacing so I can do a comparison between the two just for the heck of it? Otherwise I'll just leave the two cards in the top two slots and let the issue rest.