Just me personally, but I like the rear gearing options on the FX. That 32t is a nice bailout for when the back is troubling you, or the legs just aren't in the game. I have a 12-32 8 speed on my roadie, with a 52t largest front chainring, so I'm crossing the streams in all kinds of ways. I don't think the FXs gearing is bad at all. I'm not crazy about the shifters, just because I like the 440s better.
If you felt compelled to change it though...the rear would be cheap, as low as $20 for a new cassette. Front chainrings are more expensive, so you'd have to look it up. Probably wouldn't equal or exceed the Comp, but there's really no need to do it.
But, we kinda had this discussion in the other thread. What's driving up the cost on the Comp is the Carbon...which is my least favorite part about it.
When my wife first got her slick drop bar bike, she wasn't fond of the brifters either. (They were Sora.) The bike itself was not right, and the next day we took it back, and got one with Tiagra shifters. More fond, but still not overly fond. 6 months later however, and it's the only bike she'll ride. Her only complaint is that sometimes when shifting the front to the largest chainring when she's tired that the push is difficult, almost too difficult. (I think that's why Lance runs a friction downtube front shifter on some of his bikes.)
I don't have back problems, so I can't really give you "expert" advice. Depending on the geometry, a flat bar bike will either have more weight on your butt, with forces traveling directly up your spine, or if it is built like a more traditional roadie, will have the same weight distribution as a bike with drops when on the tops or hoods, just minus the extra hand postions and wind refuge. But, depending on your back, the flats may be best. I'm afraid I can't adequately address that issue.
If you go flatbar, I'd get the FX, and put some 700/28 armadillos on it. Then I'd go check out the trekking bar at Nashbar, currently being unloaded for $10, I believe, if the single hand postion gets uncomfortable. $15 more on handlbar wrap, and you're set.
For me, depending on the day, between 8-10 miles is the cutoff for a comfortable ride on a flat bar bike. Depending on geometry, the butt starts to hurt, or the hands and wrists start to hurt. Just me though.