The salesman was wrong about a 53 plate cooper S having 170bhp, it has 163bhp.
I have a Bluefin on my MCS and it has made the the engine so much smoother. I don't know if it has an extra 8bhp, but it does feel more powerful than before. It's also more economical as well.
When i first uploaded the new Bluefin map onto the ECU, it didn't feel any different to start with, as the ECU has to learn how to use the new settings. But after 50 - 100 miles it felt way better than before.
Once the bluefin is loaded it stays there until you reload the stock map back again.