usb3 vs thunderbolt
Dont bother with external thunderbolt I have MBP 2013 with usb3 and thunderbolt2 after experimenting with external hard disk drive 1 TB usb3 it worked satisfactorily on small projects but when rendering large files it lagged and caused fcp to become glitchy slightly but them I got an external 120gb ssd and it worked perfectly for rendering, make sure you get an ssd with 500mb/s read/write speeds. total cost should be around $120 US the speed difference between a thunderbolt external drive such as lacie is negligible. The speeds of thunderbolt and usb3 are only what the cable and computer can deliver but real world speed to an actual device are much slower expect around 250mbs/s for usb3 external ssd, make sure you plug it in using the supplied lead, I bought a aftermarket connector and then picked the best drive I could afford, this allows you to interchange ssd's as required rather than a complete unit which are more expensive but exactly the same or slower. Dont use an extension usb cable ie. male on one end and female on the other it reduces the speed back to usb 2. Transfer between thunderbolt to other devices may be faster mac to mac but network rendering is hard to set up correctly and fcpx doesn't allow for it.
Compressor wont do it either unless configured correctly.
This is what I have found anyway.
I hope they finally develop an external graphics card to make full use of thunderbolt. Hopefully that would enable faster rendering without the need to buy a Mac Pro or HP Z820 cheers