To answer your question directly, 144mbit application throughput is a bit on the low side. I usually see around 300-400mbit from a 3x3:3 client to a R600 or R710. Are you sure you're on 5GHz during your testing? What you're describing seems like near the ideal limit for 2.4GHz.
To be perfectly honest, the upgrade from 2x2 MIMO to 3x3 MIMO often doesn't translate to as much of a real world benefit as you might think. Even "3 stream" laptops often thoughtlessly place 3 antennas near each other for the sake of checking off a checkbox, not adding meaningful spatial diversity.
Also, quite honestly, the analog side of the R500, R600, and R710 series (from the teardowns, the antennas are very similar in this line) is noticeably better than the R700 series (which is essentially a 7982's antennas on an AC capable radio chipset). I never had a situation where my R600 was beat by a R710, and performance even between R500 and R600 is very similar for the wide majority of clients I have.
Are you running standalone? Standalone 22.214.171.124.432 is a newer release than 9.8.3 for standalone: https://support.ruckuswireless.com/software/838-standalone-ap-software-100-1-0-0-432-indoor-aps
and it is available for all users.