I haven't put an SSD in a USB3 enclosure, but I have tested them in a USB3 dock.
The fastest transfer rate was 430 MB/s with the following setup.
Intel Native USB3 on Z77 with Thermaltake BlacX 5G USB3 dock
Windows 8 which supports UASP over USB3 (supports NCQ over USB3).
The dock/enclosure also needs to support UASP.
If you don't have UASP support then forget about connecting an SSD via USB3. Without UASP support it's just a waste of a good SSD, IMO.