So what advantages (if any) do you think the Sabretooth will have over the Gene V with its larger form factor?
More space and more slots....about all.
As for can you SLI/Crossfire on a Gene? Sure you can. It'll fit two 2-slot video cards just like any other motherboard with the PCIex16 slots spaced two slots apart......meaning with a slot between the x16 slots.
With watercooling an SLI/Crossfire setup on the Gene, the space would become a complete non-issue as typical water blocks for video cards are thinner than the OEM air cooling solution already present, and the air cooling solutions fit the Gene.
I do know two 7970's fit a Z77 Gene from personal experience. Was toying around with XFire a few weeks ago. Moved back to a single card but two fit perfectly, and I wasn't watercooling them. Watercooling...they'd fit with room to spare. Of course, you do lose access to the lone PCIex1 slot, but you do sacrifice to play with mATX boards.
My current setup is inside a PCLabs QMicra V2 case, so a full sized ATX board wasn't in the cards as the case only fits mATX boards. But I don't regret purchasing the Z77 Gene at all......rather like it a lot, actually. Simple OC'ing my 2600k to 4.6GHz without a sweat or work.