This is what I get with the 1tb Samsung T3 - I also have 500gb T3 and the performance is the same. The T5 is supposed to be slightly faster but I don't know that it would make much difference on the Mini.
I have never installed an internal SSD on a Mini, but I'm sure someone else can comment on that. I think the 2014 Mini could get read speeds of around 700MB/sec (this is what I get on my 512gb 2013 MacBook Air) however that requires the correct kind of SSD and cable. I think these are more expensive than the one in the link you posted.
But you run the risk of damaging your Mini by opening it up - it was never designed to be user upgradeable. Even if you don't do any damage, you will void your Apple warranty. I wouldn't want to do that with a brand new machine.