I asked an OWC rep in an online chat about the speed difference between an internally installed SSD and an SSD in their On-the-Go USB 3.0 enclosure. His response:
I also called Apple sales support. The rep told me that installing an SSD will void the Mini's warranty. I guess if there was a problem requiring warranty service, you could put everything back the way it was, but there is a risk that it will not be covered.
Jeff
It will be about twice as fast internally as in the On the Go USB 3.0. Normally USB 3.0 is slated to go about 480-500 MB/s, but as it is so new it only goes about 300 MBps or a little slower. Until the USB 3.0 standard is fully updated to the 500 MBps you will get better speed internally; it will be about 550 MBps in your computer.
I also called Apple sales support. The rep told me that installing an SSD will void the Mini's warranty. I guess if there was a problem requiring warranty service, you could put everything back the way it was, but there is a risk that it will not be covered.
Jeff