My 2011 mac mini doesn't support metal for el capitan apparantly. Any how even after googling I still don't understand. What is metal? What exactly does it do?
Assuming your needs stay the same Mini should be good enough for several years.
If your sig is correct and your Mini only has 2 Gb of RAM I suggest upgrading it to at least 4 Gb. While it will work with 2 it is the bare minimum required for OS and it would work faster with more RAM. RAM is very cheap and single stick of 4 Gb will only cost about 30€.
But this has nothing to do with the performance improvements. If anything, Metal acceleration in Core frameworks will improve efficiency, but not performance. Improvements in Metal come from more efficient batching and render command dispatch, and there are not many of those in UI to begin with.
Ya sorry forgot to update but have now. 2gb was not cutting it shortly after I got her. So I baught an 8gb stick of ram for the mini and have been using 8gb for years now.