It looks to me like pci-e x8_3 is split into x16_2 and x16_4 with the NF200 splitter chip if you use either of those slots, effectively disabling the x8_3 slot.
It is possible to run both gfx cards on ports x16_2 and x16_4 and put the SAS card in x16_1. This will result in a lower pci-e lane count between your gfx cards and the cpu, but most reviews indicate that the every day effect is slim to none. So in your case this is what I'd try.
Asus ROG G55VW:
CPU: i7 3630QM
GFX: Geforce GTX660M with 2GB ram
RAM: 2x8Gb 2133MHz Kingston HyperX Impact (surprisingly the G55VW uses the 2133 X.M.P. profile)
Storage: RAID0 with 2x 256Gb Crucial SSDs (both msata, one in a sata adapter)
OS: Windows 10 x64