Yeah, the current Mac Mini is a no go with UHD at 60hz - not possible.
To be clear, I am not saying that the current 13' rMBP will drive a 5k display at any scaled/ downscaled resolution. According to Intel's product page, the Broadwell U series processors used in the current 13" rMBP (any configuration) will support a monitor with a native resolution of 3840x2160 at 60hz. That includes using one of Apple's scaled resolutions to simulate 2560x1440 (HIDPI). In that scenario, OS X tells the GPU to renter the desktop at 5120x2880 and then down-sample that massive image to 3840x2160. And all of this can be done at a 60hz refresh rate.
Here is the Intel page I am getting my display support info from (see the GPU section):
http://ark.intel.com/products/84985/Intel-Core-i5-5257U-Processor-3M-Cache-up-to-3_10-GHz
[doublepost=1472940834][/doublepost]On second read, I think I misread your post... To answer your question more directly: yes, the current rMBP will support 2560x1440 HIDPI to a 3840x2160 display at 60hz.