Hi Rod, OK if my maths is correct everything checks out pretty good. The only thing that is amiss is that your kernel speed no way needs to be 100KHz. The plain vanilla 25KHz setting is more than twice what you'll ever need from the info you've posted.
Whilst a 64V supply would give the "optimum" torque/speed curve for your motors, as you never go much above 1300 full steps/sec your 42V should be fine (according to the torque curve for your motors - different driver but probably close enough). Plus of course your driver's max voltage is 50V anyway.
JFI - are you sure your steps/pers are correct for all your axes? They're slightly "funny" numbers. X is CLOSE to an inch lead, Y and Z are CLOSE to 5mm lead and A is CLOSE to 1mm lead.
Anyway set that kernel to 25KHz and see what occurs.
Ian