I got the mini about 3 yrs ago, but have only just put the SSD into it(end of last month-got a tech guy to do it, didn't feel happy doing it myself-had a look once decided NO!!!).
I ran the OS for a long time (2.5+yrs) from a Fw800 with originally a V4 256Gb then the M4, that worked surprisingly well even though a lot threads said it would be slow when I was doing my research.
The main difference I see comparing the two setups is the boot times with the Pro being twice as fast, but day to day use there is not a hugh difference. Yes Blackmagic show a BIG difference (267ish to 107ish)but thats not real world use.