Having read through all this thread I decided I needed to see for myself how my 13" TB MBP (8gb ram, 512 ssd) would cope with the 5k.
My local Apple Store didn't have any same spec Mbps they could use but were really helpful and happy for me to bring mine in.
Having played about with it this evening I've come to the conclusion it just won't cut it for using Lightroom. Browsing my library is fine, the image quality is nothing short of stunning, but most of the tools in the develop module just lag too much to be usable. The most noticeable was trying to edit a custom tone curve - almost impossible to do due to lagging

I tried all different combinations of tool and image placement to no avail.
I did find the rest of the experience perfectly acceptable - browsing, dragging windows, exposé etc all worked fine - I just know I couldn't live with LR being like that.
The rather large fly in the ointment was that I bought my mbp at the end of November. However, all credit to the team there, they had a chat with their manager and have agreed I can return the 13" and swap for a 15" if I want to even though it's well out of the return period.
Just to be sure I'm doing the right thing, does anyone with a base model 15" have experience of running Lightroom with 16megapixel DNGs on this monitor? Will I be guaranteed a good experience if I upgrade the MBP??