I'd rather discuss it here... I was hoping I'd hear something from a hobie representative.
Most racing cats these days are being designed around Formula class rules.
So under Formula rules each manufacturer builds their boat under the class rules and in theory all boats should be equal in speed and it then comes down to the sailors ability which is great for one design racing. Great for the sailors also because we can now all race against each other on the same boats although there are different manufacturers building the boats. We can all get along.
If you're looking for a great speed advantage with one boat over the other I don't know if you're going to find one, and that's the whole point of the Formula class. So then you have to look at the details and quality of the boat being manufactured. Unfortunately I haven't been on an I17 or for that matter even seen one up close except in pictures so I don't want to comment on what I don't know. I have been on the FX1 only briefly and in light air, but enough to be impressed with the boat. It sailed very easily, the steering was responsive and light, the boat tacked quickly with the flat bottom hulls, I was able to trap in around 7 knots, all the controls were easy to reach from the trapeze and I think if the wind increased the boat would be easy to depower with the downhaul and twist of the square top.
As for the hull shape, I do only have limited time on the FX1, but I've have quite a bit of time on the Fox which has a very similar hull shape. As I said above, one of the main things I notice with the hull shape is with the flat bottom the boat is much more responsive, similar to sailing a small dinghy. I think the boat sails much different than say the Tiger just because of the shape of the hull, and there is a whole new learning curve to adjust to this. Sailing the boat flatter and not steering as much is part of the learning curve, but it's not too dificult to figure out. Downwind even if the bows would bury in the waves it never even came close to the point of pitching.
Gotta Go, but if you have any more questions or something specific to ask I'll respond tomorrow.
Hobie Cat USA