2.5" Form Factor drives are bus powered so none need an external power brick. USB 3.0 std calls for .900 of an amp which is more than enough for any 2.5" hdd. :)
As for speed, it depends on the controller chip used inside the enclosure. The JM based ones are extremely less than optimal, but the ASMedia based ones are VERY good. They could have been your bottleneck MM, plus up until only a couple months ago the NEC controller used to give USB 3.0 to systems (PCIE adapter and motherboards themselves) had a performance wall of about 145mb/s or so. A firmware update fixed that...but not all companies have implemented it. I get my firmware updates from:
SONY/NEC firmwares
Renesas purchased that part of NEC (or so I heard) so the later firmwares will show your devices as a "renesas" controller and not NEC. ;)