Titan X compatability with older 2688 core Titan's when rendering with Iray.

I just finished speaking to a support rep at Nvidia, and he suggested that I ask here for advice.

I currently have 4x GTX Titan (2688 core) on an Asus Z9PE-D8 WS With dual Xeon E5-2687w, for rendering with Iray / Mental Ray.

I am wanting to replace the primary card with a Titan X, and keep the other older titans to help with GPU rendering.

The question is, can I use a Titan X and older 2688 core Titans at the same time when rendering with iray?

I have read that trying to use a an older Titan and a GTX 580 together in iray produces an error, so will using a Titan X and 3x Titan 2668 core together also produce an error in iray?

I am using the latest version of max 2016, which I believe already has the Maxwell Patch applied.

My O/S is Windows 7 pro x64.

I am aware that SLI will not work between the different chip sets.

I am also looking for any benchmarks that show how much faster the Titan X is compared to the older gen GPUs like the Titan (2688 core) 780ti etc in Iray. It will help me to decide if its really worth the upgrade.

Is it possible in one of Nvidia’s workshops/labs to put a Titan X and older Titan in the same system, to see if they can both be detected by 3ds max, and render together in Iray without errors?

Would be really helpful.

I am aware that using the older Titan’s with the Titan X will limit the Titan X memory to 6gb.

I plan to eventually replace all the old Titans with Titan X, but only if the benchmarks justify the cost. I don’t want to end up replacing all the Titans with Titan X only to find that an 11,000 iteration render on an interior architectural scene that takes 15 minute to render, is only one minute faster. It really wouldn’t justify spending £3500 on four new cards. So if there isn’t much improvement over the old Titans, I will only want to replace the primary card for connectivity reasons, but that’s only if its compatible with the older Titans in iray.

Does anyone know where I can find the information I am looking for?

Many Thanks.