For what we know, it's foolish to do such a severe asymetric crossfire, it will be useless in all current games and the result in future games is unknown but probably next to useless unless proved otherwise. Ideal result is R7 260X + R9 290 or 7950 behaves as two R7 260X for graphics performance, with additional horsepower used for gaming GPGPU tasks, but it might not be the case.
A lone 290 would be just much easier.
The question was not if a R7 260X would pair up well with a HD7950 or a R9 290 in Crossfire in DirectX.
I think that part of the question was pretty obvious, even more considering that this is the Mantle thread.
From what I know, the answer to the question is: we still don't know what kind of multi-gpu combinations and performance boosts will be made available with Mantle.
I'd guess it'll all come down to either Mantle's Crossfire will still use AFR or not. Predicting per-frame performance between GPUs with different capabilities and memory sizes will always be
very hard AFAIK, which is why nVidia doesn't allow SLI between anything other than two identical graphics cards.
The cherry on top would be to have Mantle natively (therefore
flawlessly) support something akin to
Lucid Hydra's solution.
That way, a R7 260X + R9 290 with normalized clocks could be
similar to a single GPU with 54 GCN Compute Units, 216 TMUs, 80 ROPs and 6GB of memory and the 260X would give a healthy ~30% performance boost to the 290.
Load balancing on the CPUs and its induced latency would probably be the hardest thing to handle, of course.
AMD would benefit greatly from this, especially for having their APUs to significantly boost the performance of mobile discrete GPUs.