Forget about that, the fastest storage device and the one where you should put the rootfs on is the internal eMMC. Using an USB disk (be it connected to the crappy onboard SATA bridge or any more suitable in an external enclosure) will always slow things down.
Booting from SATA and using an SSD is a great idea if you buy a board that features SATA (like the way more cheaper and in most areas also faster Banana Pi M1) but on boards that use the horrible GL830 bridge it's the worst idea. The interal eMMC is way faster than any disk and especially any device connected to the 'SATA port': http://forum.armbian.com/index.php/topic/474-quick-review-of-banana-pi-m3/