it is not well known, but some of the 48v controllers are actually the 72v model, depending on who you buy them from. there is at least one vendor who deliberately underrates his controllers to avoid people using them at the higher voltage. have seen a 35A controller with the 55v irf3205 devices in it, but most do have the 100V irf4710.
the latest 72v controller i bought has 160v electrolytics, some of the 48v ones i have seen had 63v or 100v. i think it has changed a bit over time, so you really never know unless you look.
the 40A and 35A are identical, the change to the shunt was made to let the X5 motors have the extra current, the capacity was there already in the 35A controller. As for it just being 2-20A controllers, that is really not quite true. the 20A and 35/40A use the same low voltage regulator, controller chip, and pwm chip and a similar current sensor, but the drive to the fets is a bit different. There was a controller once that used two separate controllers in parallel but this is not it.
when you put two fets in parallel you add up the gate capacitance and the charge it takes to turn the fet on and off. the driver chips used in the 20A controller don't have quite enough capability for the PWM switching of the two high side fets, so they added a discrete transistor buffer stage after the ir2101 driver in the higher current controllers. this stage seems to have enough drive capability to handle even better fets than the 4710 they are using. i have switched in the 4310 and it works great.
-bob