My question, is do you really NEED 10gb connection between your computers? What are you trying to get out of your setup? A 1gb connection is theoretically capable of 125MB/s but really you are probably looking at 100MB/s at the actual top speed. That's still a ton of data. Your best bet, is to hook all three computers up via 1gb ethernet (since that is the max networking option they both have) and network share the R6 connected to the iMac to the MBP. If that isn't fast enough, then get each one it's own R4 or R6. However, the problem with this, is that you would have copies of everything stored on two different RAID arrays which can be a lot to juggle and figure out.
Are you sure you realllyyy need more than a 1gb connection?
A question for all, how "good" is the switch in the Time Capsule? Most of the consumer grade routers/switches that I've used do not allow me to really hit max throughput and instead I have to go Prosumer/Business grade switches to really get maximum speed (I have an extreme, but only use it as a DHCP and wifi access point connected to an expensive switch).