7 GPU set-up

Hello,

I am investigating a set-up for GPU password cracking.
The GPUs in Chassis B would be connected via riser USB3 cables.

Chassis A (rackmount):
GPU: R9 290X (x3)
CPU: i7-4790K
Case: Chenbro RM41300-FS81
PSU: Lepa G1600-MA 
Mobo: MSI Z97 GAMING 5

Chassis B (rackmount):
GPU: R9 290X (x4)
Case: Chenbro RM41300-FS81
PSU: Lepa G1600-MA 

(+ cooling, + RAM, + risers, ..)

Is the motherboard fit for this? Would the CPU be able to support the 7 GPUs? Is it possible to go beyond 7 GPUs (by using splitters)?
Is there anything else that I'm missing that could cause problems?

I'm trying to go for the most cost-effective solution, alternatives are most welcome! 

Many thanks,
Cedric