I did it.
I bought the 2.5" Samsung 850 Evo 250Gb, cloned my mac onto it, and installed it in my mid-2010 MBP.
The speed improvement is enormous, it's pretty close to the speed as when I first got it.
Thank you to everyone for suggesting it. It cose me AUD $180 all up, including a SATA-USB adapter.
What did you have before? Spindle drive or smaller SSD?