I set up a 6400 sq ft property a while ago using Unleashed (R500, 600, 710), and I definitely noticed a big improvement when I replaced an R500 node with a R600 and the R600 root nodes with R710's. Throughput increased by about 50%.
If adding another mesh AP is not an option (which, BTW, is my preferred recommendation), then I think your next best option is to use a pair of R710's: One as a root and one as the distant mesh. The extra sensitivity and the 4 spatial stream capability will give you the best shot of a reliable mesh link.
EDIT: But again, if there's any chance that it's possible to reconsider adding an AP between the concrete walls, it is worth considering.
Also, in case you haven't tried yet, try switching your 5GHz channel. In one of my environments, switching from channels in the 36-48 range to channel 100 delivered a noticeable performance improvement and stronger mesh signal. Not sure if that's due to the regulatory transmit power differences, radio sensitivity differences, or propagation differences caused by the frequency change. But it's worth a shot.