The 450m 5GHz is 14x14 BTW... and is considered 'massive' MU-MIMO, an industry first.
In regards to the spectral efficiency showing 50 bp/Hz for the 450m 3GHz vs. 40 bp/Hz for the 450m 5GHz, this is not accurate. The 40 bp/Hz figure I would guess is from an original spec sheet and does not take into account the many optimizations that have been made since its original release, along with the addition of uplink MU-MIMO that was recently added in firmware version 16. If anything, the 450m 5GHz should have a higher spectral efficiency due to the extra MU-MIMO chains. I know for a fact that the 450m 5GHz has a higher aggregate throughput then the 3GHz model for instance.
The 450m 3GHz is 8x8 mostly due to physical constraints in building an antenna array at that frequency, along with size and expense of the additional power allowed for licensed 3GHz use. Using an 8x8 array also allows for it to be used as an LTE RRU with a firmware update in the future. Current LTE advanced RRU's typically revolve around a maximum array size of 8x8.
And yes, more MU's are better, but as mentioned above, there are many factors to consider when designing a MU-MIMO platform when you want operators to adopt and deploy. Make it too big, expensive, complex, or use too much power, and few if any would deploy it.