Help in understanding MIMO
I'm going to try and answer the latter half of your question because I kept on running out of comment room:

MU-MIMO offers improvement because most mobile devices (called User Equipment (UE) in most documents I've seen) don't support as many spatial streams as the base station. In SU-MIMO, only one UE can transmit at a time. So if you have a 10x10 (10 upload x 10 download) base station, and a 2x2 client, the base station can only use two of its spatial streams. This difference is illustrated from this white paper:




So to return to your example, if both UEs and base stations are 2x2, your logic holds. However, if each UE is 1x1 OR if the base stations is 4x4 then MU-MIMO is twice as fast; 8 vs 16 ms for the former and 4 vs 8 ms for the latter. 

As someone with a passing interest in MIMO, I'm not sure I'm qualified to take a stab at the mathematics of MIMO, however, I highly recommend you read white papers or application notes from manufacturers of chips or test equipment first. Although they're written to encourage you to buy their product, they often give more background than academic papers because the people buying the equipment are often not experts (yet):


For a very high level (maybe too simplistic) overview look at this Keysight Application note
The Rohde and Schwarz whitepaper I referenced above is a little more useful for learning the math
This technical journal article has a few useful points as well
Here are two more academic articles which I was able to parse most of, one and two