I personally went for bluefin, which I received 3 days ago. Best money I ever spent. Not the best time of year to get the best from Bluefin, as the roads are always wet, and its hard enough to get the power down in the dry. But, I would absolutely recommend it to anyone. This car really flies with Bluefin. Some people have said its not much different up to 3k revs, but I notice it just about 2k revs. You only need a light touch on the accelerator to feel the difference. And as I've only had it for a few days, it gets better every day, as the car gets used to the bluefin.
And fuel economy is unchanged. Not too surprising I guess, as you're only using the extra power occassionally.
Anyway, just sharing my experiences.