Not all SM's are at MCS15. I think the biggest reason is becaue most of our links are NLOS, which typically affects one chain more than the other. So, even though the combined RSSI is -55, the chains may be something like -56 and -66, which can pull the MCS down a bit. But on average, most are MCS15 or 14.
We use the KP-Performance antenna with the built in enclosure.
We keep them under 20 SMs per AP.