I'm about half way through a tank with a 200:1 mix in and it does seem to make a difference, it's a bit smoother and it feels like it's got a bit more power - I'll put a couple of tanks through and see how it feels then.
I was not expecting an immediate difference so I'm quite impressed, I have gallons of semi synthetic from when I had a motorbike shop so it's nice to find a use for it!