I've got a Giant Cypress SX which is fairly similar to the old 7500fx. It's outfitted with fenders, rack, panniers, etc. All loaded for work, it probably weighs about 40 lbs. Even though I have a nice road bike which I don't use for commuting, comparatively I thought the Giant was pretty speedy. Then I bought a cyclocross bike this spring. It has no rack, fenders, etc. On the days I ride it (decent weather forecasted), I carry all of my stuff in my messenger bag. It is way, way faster, even with cyclocross tires on it. Only drawbacks to it are no rack & panniers, so I have to carry my load on my back, and no fenders, so if it gets wet, I get skunk butt.
Personally, I think it's best to have more than one bike for commuting, even if just for variety's sake. They all have their advantages in different conditions. Except for the FS MTB. Not much use for that in the city unless you are trying to get a 20 mile road bike workout in 5 miles. The 7.5fx (Jamis Coda line is better IMO) has the advantage of being able to load it up with rack/panniers/fenders (the latter probably not needed in PHX), so the weight is off your body. CX has the advantage of being as light as a road bike. With both, you can hop curbs and ride through dirt roads without white knuckles (with decent tires). Another advantage to the CX is that you can get an extra set of wheels/road tires, and it becomes a very capable road bike. Hard to call a flat bar bike a true road bike.
Personally, I wouldn't ride an expensive road bike to work. First, it might get stolen. Second, you'll wear it out during mundane riding. Save it for the fun rides. Not that it's not fun to ride to work (that's a big part of the reason for doing it), but it's like commuting in a Ferrari vs a Civic. I figure if I rode my nice road bike to work, I'd never feel the need/desire to take it out for a non-commute ride.
Ride the road bike for the time being. At the same time, methodically watch Craigslist for bikes of interest and be ready to buy. Advantage to buying used is if you don't end up liking your choice, you can probably sell it for the same as you bought it used. New you lose 25-50% almost immediately on resale. Or go out and buy new ones if you prefer. My only beef with off the shelf CX bikes is that they are mostly aluminum these days (exceptions Jamis Nova, Bianchi Volpe). Since you already have a CF road bike, you're going to find aluminum a harsh reality. I'd look around for a steel one, though it could be a bit of a wait. On the other hand, many people think steel is passe, so they are not quite as sought after as an aluminum CX is.
My Waterford CX was listed on Craigslist for a few days before I snapped it up. Lucky me. Then again, I've seen a brand new Ritchey Swiss Cross bike listed multiple times (still listed in Denver!).
Best of luck whatever route you choose. Bikes are cool!