Sure thing.
Actually a user posted the same tutorial I followed back in December, I built up on his post by adding how to put a SWAP partition as well, which makes the Pi able to open several chromium tabs, a bunch of documents, etc. Without worrying about freezes caused by running out of RAM.
So the way I understand the boot process is that there are two disks a root partition and a boot partition. The Pi 2 needs the boot partition to be on an SD card (apparently the Pi 3 doesn't but they need something that hasn't been developed to make it work).
The boot partition if I'm not mistaken runs the kernel, and then mounts the root partition, the root partition can therefore be anywhere you point your cmdline.txt to (inside /boot/cmdline.txt). So... you can therefore offset your root fs to a different drive on USB. Even an SSD is a great improvement over the crappy IO of an SD card, plus you get to not worry about random corruptions of SD card.
You get this improvement because in my experience the SD cards I use get to tops 11-13MB/s write speed... whereas the SSD gets to 30MB/s, which is much better, despite the fact that we can't take full advantage of the max speed on the SSD due to USB2.0 port (hopefully the next Pi has a better IO......).
Anyway, if you are interested here's the post I was talking about, the adafruit script works well for mate too.
In conclusion, it is almost as simple as copying the root partition to an ext drive. Just make sure that the drive is mounted during boot and also point your cmdline.txt to that drive so that it mounts that root fs instead of the default one on SD card. And then you are done 
Cheers!