I think there's a slight bit of hyperbole in that statement. The firenze isn't designed to be stiff, in fact, certain bits of its geometry is designed specifically to be compliant. I can feel it flexing a bit when I'm really laying it down (which I don't do that much on long 100+ mile rides) but that's like saying you wanna get a new stem to save 40 grams when you can easily drop several hundred grams by peeing before a ride. I do enjoy the plush ride that it offers, which you can definitely feel that it's probably one of the plushier rides out there. Like I said before, it's solid, not stiff. If you want stiff, I think al is the best bang for the buck.
With regards to titanium, I've heard that they recently have solid 1 piece tubing made of 6/4 titanium. Not sure if that's in your price range but just in case you open up your wallet to 6/4, some companies use folded sheets of 6/4 welded at the seam to form a tube and some use the newer real tubes. With the folded pieces, you're just getting the structural strength of a folded piece of sheet metal with a weld along the fold. Not sure if that means anything now but it might in 50 years. I wouldn't worry about it though, if you do go titanium, I give you about a year or two before you feel the itch (the good kind) to try something different too. I love my firenze for going the distance but for racing and pure climbing, I'm currently looking for new wheels but might eventually also get a carbon alongside the titanium.