I've noticed that the max bank angle used by the AP in flight seems to vary based on the amount of time compression in use. For example, if in a max bank at 4x compression, and you reduce to 2x or normal, the bank angle increases, and so it appears the rate of turn. Is this true?

The NGX is not aware of any simulation rate increase, the auto time compression feature was first released with the later PMDG 777.  If there is any correlation between bank angle and simulation rate then I would suggest it is unintended or anecdotal.  The rate of turn is related to the bank and the TAS, and should not have anything to do with simulation rate or type of aircraft.