hi rob!
1.) well, heating is an issue, mostly when the drive is NOT running. BUT - it only occurs if the drive is enabled/powered up. this can be controlled both for the DRV 8825/RAPS 128 and the phidget 1067 - and the drives should be disabled when they are not running as long as you don't need their holding force. the bigger issue is the heating of the driver chips. the phidget board solves this in an elegant fashion, the DRV and RAPS need massive cooling bodies. i doubt whether this problem on the chip has a positive effect on the life expectation of those things ... i never had problems with hot motors when driving the telescope, but i had hot drives on the bench when not running them
2.) from my experience, the only current that you can run the system on is the current that allows for smooth operation under the given load. this is easier with the phidget controller as you can control the current via software. for the DRV / RAPS, this requires turning a little pot ...
3.) when using a non - RTOS like Raspian, you NEED motor controllers with interfaces because you need a microcontroller to drive motor as a realtime application. i remember the interfaces with RS232 when i was a young guy, we drove them with a microVAX! fortunately those days are over ... BUT the phidget 1067 is controlled via usb, this is how TSC works. there is an interesting driver that already has a microcontroller on chip and can be directly addressed via spi - STMicro L6470 dSPIN, and there is a breakout board from spark fun. i made a pcb that features an arduino mini pro and 2 DRV-drivers and also has connectors for controlling the arduino via spi ...
4.) well, you may have seen my video on that thread - this is 350 kg moving at a rate of ~ 0.7°/per sec in two axes. the motors are set to 1.5 and 1.7 A, and the are powered by 12 V - let us assume a power consumption of ~20W per drive. No heating problems on the drive. Big heating problems with boards other than the phidgets as the thermo protection switch on the driver chip drops with in seconds if you don't take additional measures (cooling bodies, ventilator...)
yours
wolfi