Hey dude,
I own a 2005 7200 FX and I have to agree that the term "Fitness" is misleading. On the Trek page and pics it looks like the FX series bikes are very fast, but the sad truth is it's only as fast as the roadies if you spring for $1000+ top of the line models. The lower ends are pretty much slow and causes you to work harder inorder to get "fit"
*Funny story that I raced with my friend's Giant Cypress SX (a true road hybrid) and got my ass spanked really bad
Ever since then I've been obsessed with finding solutions to modify my FX to over take the my friend's Cypress
I've also ran into similar problems comparing my Specialized Hardrock mtb with knobby tires against my gf's Fuji hybrid with 700c tires. We were puzzled by how much faster the Hardrock was and ended up just getting her a Hardrock since it seemed to work for her in terms of speed. The best analysis I could come up with lies in the GEARS. The hardrock just had smaller chain rings and generally lower gears allowing the cyclist to accelerate quickly with closer ratio gears. The hybrid does have higher gears, but the main problem is that the gears are so far apart it actually slows down your progression of acceleration. So in theory if you have a strong enough motor you can over take the mountain bike at the highest gear...
I've identified the weakest links of my bike (if your intention is to go faster with less effort).
1) The tires: These bontrager pieces of crap really have to go. 35c is too wide if you want to go faster plus the thin threads do a decent job of creating enough resistance to slow you down. I upgraded to 28c road slicks and immediately noticed a world of difference in effort and speed. I believe optimal should be 25c if you're going to change your tires.
2) Gears: Hybrids in general have too wide of gear ratios so if you aren't in the best of shape you would have trouble transitioning through higher gears. It's difficult to explain gear jargen, but in a nut shell if you just get a cassette that has closer ratio gears you'll notice faster acceleration. An example would be 12-23 or 12-26 in a 8 or 9 speed cassette.
3) Wheels: These wheels actually aren't too bad, but upgrading would further improve performance especially if you opt for the bladed spokes on the upper end Bontrager wheelsets that reduce drag by 40%.
4) Weight: Obviously the lighter the better so whatever you can swap for carbon or just lighter material at a reasonable price you should do it. Currently I have a carbon seatpost w/ lighter race saddle and looking to swap the handlebars, stem, and eventually the fork.
Hope this info was helpful!
Cal