I would expect an FX to be pretty different. Your current bike is a 'comfort ATB', and you'd be moving to basically a road bike with flat bars. You'll be on bigger wheels with narrower tires and be riding with a much less upright posture. I'd be surprised if you didn't feel a significant difference between the two. However, I think you should just go test ride one and see. Just go see what you think when you're on the bike.

As for 7.1 vs 7.5 etc., the differences are not nearly as large as the differences between the FX and your bike. The higher end bikes have better components, but the basic feel is the same. I have a 7.3, and I bought it over the 7.1 and 7.2 for the better shifters, derailleurs, and so on. (Oh, and the better tires.) It felt pretty similar, though, perhaps smoother shifting. The 7.5 I felt actually rode better, presumably due to the carbon fork and rear dampener, but it wasn't a night and day difference (and it wasn't enough for me to justify the upgrade).