signal to noise is 54
Ok. MCS12 is 16-QAM, where MCS13 & 14 are 64-QAM. To have 13 & 14 working stable you are looking to achieve signal levels in the range of -35 to -45. With an average general background noise level of -105 you are looking for 60 to 70dB S/N.
Actually I have a 9km link with two 30dBi Jirious H+V pol. antennas on both ends and rb911G at both ends (same board basically as your 912).
I have the same issue as you; MSC12 is nice stable with a connection rate of 180Mbps (40Mhz channel and SGI) and a CCQ of close to 100%.
The moment I force both to work with MSC13 or 14 the CCQ drops considerably and the net result is I can 'pump' less traffic over that link (and have more unstable ping time) than in MSC12. So I set 12 fixed and a fall back at MCS10.
My tx/rx signals are -51/-49 seen from AP-bridge unit and only the tx has a fixed CCQ of 100%. The rx which in fact has slightly better signal is continuously running between 97 to 100. (This is actually the 'upload' stream, which usually has less traffic and links with low traffic are always prone to lower CCQ levels.
Ping times are pretty stable around 3ms with an occasional max of 10ms.
So, my link is much shorter than yours. But although my frequency is relative free, there are other frequencies in use just outside the band I set for these units.
I don't know about your link, but I'll guess you have a much more free spectrum that would explain why your link performs as good (or as bad) as mine while you have to bridge a 3 times larger distance...... I don't think you are doing so bad after all!
Like me, if you are really thinking of getting a higher throughput, or a more stable link, I think you might start considering to replace your boards for the new ac boards. I am even thinking of converting it in a triple chain link. This link for me has to start transporting way over 100Mbps in the near future which presently is not achievable.
You have to realize that given speeds (and link stability or latency) are a product of the distance, antennas, radio output and receivers sensibility, free fresnel and state of the used spectrum. Your issue is the sheer distance, mine is the noise level and interferences from other links.
One other think you can do to improve the link is to get yourself bigger antennas. Rocket dish come in 30 and 32dBi models. And the new 30dBi models of MT also look very promising.