If you own it, then install PMS and let it work, watch the dashboard while people are using it and you can see what's going on.
It may be fine, it is just difficult to gauge that unless someone else has done the specific thing.
I started with a Core2Duo Plex Server in 2009 and it has grown to an 8Core 30TB w/ hardware decoding machine.
So the mini did serve my purposes for quite some time, mainly because my family that would stream would never hit it all at the same time, and high bitrate files weren't a thing then, a 6core Mini is also more powerful than a dualcore from '09.
So if you have one, there is no reason to not try it out and know for sure.
If you are going to buy a machine that is for Plex, I would, myself, look elsewhere.