External fast SSD for multi-cam clips?

I've been using a early 2011 15" MBP with dual SSDs inside, by removing the superdrive.
Now, since the gpu started acting up I'm thinking of buying a new retina 15" 2014 MBP.
I use a SATA II 240GB SSD (Vertex 2) for the OS and apps and a SATA III 512 GB SSD (Samsung 850 Pro) for the media files. The second SSD served me well, because I use a lot of multi-cam clips with around 10 angles and I need the speed the second SSD provides (around 500 R/W).
Now, the new retina 15" 2014 MBP has a PCIe-based flash drive (I'll probably get the 512GB) that's super fast, but there's no space for a second internal SSD. So, what are the options? An external fast SSD (around 500 R/W), hopefully compact and buss powered? If so, which one would you recommend, I'm struggling to find anything that fits my needs...
Thanks in advance!!!

T'bolt 2 RAID.

That fast, compact and buss powered? I haven't seen any. Would you recommend anyone in particular?
The LaCie Little Big Disk Thunderbolt 2 is great (super fast & compact), but it's too expensive (I don't need 1TB, 512GB would be enough) and it's not bus powered.

BTW, I already have the LaCie Rugged USB 3.0 Thunderbolt Series 256GB SSD, but the controller tops at around 250MB/s read and 380MB/s write.

Apparently there's a more budget-friendly version of the LaCie Little Big Disk Thunderbolt 2 out,
from Buffalo called the DriveStation Mini Thunderbolt 512GB.
It's obviously 512GB (I don't mind), not TB2 but peaks at around 700MB/s R/W speeds and is not bus-powered (I mind).
Still looking...

It seems that all bus-powered solutions max at around 380MB/s.