I get 58 mi/hr @ 80 amps for my Model S with dual chargers and 44 mi/hr @ 70 amps on my Model X..... does this sound right? I was expecting around 50 mi/hr....

Yeah, don't forget that the battery is the same size (90 versions of the S) but the range is lower. So even charging at the same amps will get fewer miles per hour on the X.

Remember that the rated Wh/mi for Model X is higher, so the number of miles gained will be lower for any given number of Wh put into the battery. Also, the Model X seems to be more sensitive to higher voltages. For me, it will back down the charging current to guarantee the charger stays below 17.5 kW. My home runs at ~250V, and while the display shows 72A before plugging in, it'll drop to 69-70 when I plug in, based on the voltage at the time - unlike the Model S that will charge at 79A @ 250V all day long.