I have a Micron (Crucial) M500 480GB in my 2014 Mini. I tested it in a USB enclosure before putting it in the Mini and it was about 10% slower with the USB vs. internal SATA using the Blackmagic test. I also have the Monster Thunderbolt SSD that estabya mentioned in post #2. Read speed is about the same as the 500GB M500, write is about 15% slower. It's slower, but I think it's because of the size (240GB) - it's supposedly also a M500. Larger capacity SSD's of the same model may be faster if they can interleave data between NAND chips. Thunderbolt 2 is 20Gb/sec., vs. 6Gb/sec. for SATA3 but for most people that's irrelevant because to take advantage of the Thunderbolt 2 speed over 500MB/sec. you need a substantially more expensive external drive.
Doing a clone, if done properly, will work.
I've put a SSD in both my 2012 and 2014 Mini's. If you search the posts on this forum, it's not difficult to find people who have broken stuff doing the work. Others find it a breeze to do. For me, I didn't find it easy but the second time with my 2014 Mini was definitely easier and less stressful because of the familiarity (despite the differences between the computers and the time between the two jobs - probably two years). If it's something that you are considering, you can also look for authorized Apple service providers to do the installation - there are people on the forum who were able to get it done for about $50-60.
Finally, a topic that always engenders heated discussion - TRIM. You can use it if the SSD is installed internally or if a Thunderbolt SSD is used but not on a USB SSD (at least not confirmed by somebody who has actually documented it on this forum). Do some research and make a determination if it's important to you or not.