I have two MacBook Pros: (Early 2009, 2.93 GHz Core 2 Duo, 4GB 1067MHz RAM, 256 SSD) and (Retina, Mid 2012, 2.3 GHz Intel Core i7, 16 GB 1600 MHz RAM, 256 SSD). The 2012 is about 2x faster than the 2009 for a simple RubyMotion Hello app on the simulator. So I'm sure the 5K will similarly smoke my 2012.
I have been considering purchasing a 27 inch iMac as well, and I have been comparing the 5K to the non-5K. Both can be configured with the Hyper-threading i7, and although the 5K clocks a little faster, I would expect the speeds to be comparable.
Big question for me is whether I need the 5K display. It's noticeably brighter, a little richer and more power efficient. But the one downside is that the current 5K does not support target display mode. Target display lets the iMac act as a monitor for another connected iMac, which would support interesting workflows with my MacBooks.