I have run the Fatty on my 300. It did seem to increase power across the entire rev range, but mostly it increased the the hit from mid to top and provided more overrev. If you are primarily looking for torque, this is not the pipe I would recommend. Although, if you are looking for a "more exciting" power delivery it would be a good choice. I bought the Gnarly because my Fatty is severely crunched, not because I didn't like it. The Fatty setup will require more energy over the course of a day if that matters in your decision.
On top speed, those RFS EXC models will always have more overall speed regardless of the pipe you choose for your 2stroke, unless you install radically different gearing.