there's nothing on maxwell except this thread:
viewtopic.php?f=140&t=44642&p=393787&hi ... im#p393787
early results there are very promising except for the latest post where the mac GPU on Titan X (maxwell) is much slower compared to the windows counterpart. i hope it's the mac environment's issue, then we would really have exceptional speeds as the Intel Xeon E5-2699v4 is one of the fastest CPU-s available (2nd overall according to passmark). unfortunately i can't download the demo scenes as the links are broken for me on the maxwell support site. also i haven't purchased v4 yet and i'm not willing to take any chances to mess up my v3 license by installing the v4 demo as some have reported.
http://support.nextlimit.com/display/MR ... emo+scenes
i would go with the octanebench as octane render is fp32 (single-precision floating-point format, binary32) as is maxwell render (mostly). also you can download it for free and test your systems. i know it's not maxwell render but this is as close as we can get to comprehensive GPU overview at the moment.
https://render.otoy.com/octanebench/res ... ingleGPU=0
furthermore, i would wait until maxwell gpu is production ready. if it will take more than 6 months, which is quite optimistic considering current development progress, we might have Volta available which is next generation and 16GB probably. if nothing else, at least freshly announced AMD Vega will bring Nvidias prices down. GPU rendering uses only GPU ram so graphics memory might become an important requirement with high res renders and textures. so more the better. memory will be limited to the smallest ram available of the GPU-s used for the rendering. one just can't add up the GPUs ram. might change in the future? don't know.
if you really can't wait, go for 1080 Ti (30-40% faster than 1080 and around 95% of Titan Xp) or with multiple 1070 if the price is of the essence. I just bought 1080 Ti because constant issues with my FirePro drivers (wouldn't recommend a pro GPU after this for anyone, also no need for much of the dual precision floating point performance with GPU rendering, not mentioning the price). if GPU on maxwell is finally usable for work, i would get another 1080 Ti or Volta. the cards don't have to match. though i would need an auxiliary, external (with ADD2PSU) or internal (aka juicebox) PSU as my current dell precision t5610 has depleted all the 6-pin and 8-pin connections from the PSU to the GPU (2x 8-pin up to 300W, my 1080 Ti is 6 and 8-pin). not an issue with non proprietary PSU-s which are quite easy and cheap to swap out.
so the power consumption is something to consider. check your PSU wattage and available connections first. 1080 Ti is 250W, 1070 is 150W. 6-pin provides up to 75W, 8-pin 150W and PCIe socket on motherboard is 75W. some have also converted 2 molex connections to 6-pin or 8-pin.