Sounds an interesting project, my guess is though that as your pcie bandwidth is already limiting your cards performance that when you use sli you will further reduce the performance of each gpu.
I say that because although you are doubling your bandwidth, that bandwidth is being shared between 2 gpu's so is essentially similar to your one card set up. When you subtract the overhead on the bandwidth that sli will impose you will have less bandwdth per gpu than you originally had, and as that was already the limiting factor you will see a decrease in your per gpu performance.
When you factor in that sli will only give 70% - 80% increase with the second card in a full bandwidth system then its debatable on how much more you will get....dependant on how much of a negative effect the sli overhead has on your bandwidth.
Still its always good to see your projects through.....you always find someone with a negative view point on the web ;-)....so good luck with that. If anyone could help you get that up and running I'm sure people here could.