Yeah the GT King and the Pro perform about the same in benchmarks, so in terms of processing power there’s not much in it as far as I can tell.
The selling point seems to be the built in audio decoding, which I have no need for as I have an AV receiver to do all that.
I run CoreElec on EMMC on mine, it was very straightforward. I don’t use the Android, so I just went for a single CoreElec install, it’s great. I’ve had to recover it a few times from a brick due to some problems with my EmuElec install on an SDcard corrupting the bootloader, which is no fault of CoreElec, but this is pretty easy. You just a USB A-A cable and follow the instruction from BeeLink to flash the original firmware.