Let's assume I would like to attach a USB3.0 external storage device to my Raspberry Pi by utilizing the SD card slot. Would that be feasible? (As far as I know the Ethernet is just a USB2.0 port in disguise, so that won't work. Not that there is any guarantee that the SD card speed would be higher.)
No, there is no way to make that work. The best you can do is connect it to one of the USB ports. The Raspberry Pi simply doesn't provide any high-speed connections.