I received in the mail a PCI-Cable extension thus I could experiment a different configuration: it was a disaster.
I have a ASUS Crossblade Ranger, thus I could actually test a 3-way crossfire. For this experiment, I use a R9 Turk GPU into slot 0, the 295×2 into slot 1, and in the slot 2 the ProDuo. In practice, I can see 5 GPUs but there is a “but”.
GPU_0 takes care of the monitors
GPU_1 and GPU2 are Fiji thus the ProDuo
GPU_3 and GPU4 are Hawaii thus the 295×2
(But) In this configuration, the ProDuo get half the bandwidth. Obviously, this is wrong in so many way that any smart person will not even try. Why waste energy ?
I wanted to make the computation configurable in order to take advantage of such a skew system even such a system. The idea is simple, if a GPU is slower, I quantify it and I reduce the problem size it can handle. If a Fiji is used in conjunction with a Hawaii, the Hawaii get 2/3 of the problem and Fiji 1/3. This will be useful in the future, but you can see the problem and the simple solution.
In practice, 4 is possible, 3 is better.