That's what my LBS said. The power transfer overrides the actual weight saving. I really like the Defy, but already hankering after n+1. They said that the Composites would not give a huge gain over my Defy and to look at the Advanced models. It's a tough call...
To be fair he has a point, in terms of weight anyway.
I have posted about Giants 3 tier carbon range previously but as I am old & decrepit I can't link it, not on my phone anyway.
Giant carbon range are
Comp = (Torayca) Toray T600
Advanced = Toray T700
Advanced SL = Toray T800
Torayca are one of the top 3 manufacturers of carbon worldwide.
Taking T600 as the basic level & T800 as the top level (in Giants range) the weight decreases & cost increases as the levels get higher.
Giant never advertise frame weights.
It is possible to get Torayca T1000 from other manufacturers & Pinerello are one of these.
My bike is Toray T800 (not a Giant)
A cycle manufacturer can call a bike T800 (High Modulus) if they use some T800/HM in the frame lay up, but doesn't mean it is made entirely of T800/HM.
In fact, having read many articles in bike mags from respected builders, if a manufacturer built a bike 100% from High Mod or Super High Mod carbon it would created an extremely light but also an extremely brittle bike with no comfort at all, the bike would not absorb road shock very well.
The upshot is that I estimate the weight difference between your Alu Defy & the carbon Comp to be quite low, 200g > 250g maybe in a like for like frame sizes.
A guy on here or maybe it was BikeRadar posted somewhere that when he measured the weight of his Giant Comp, he was surprised at it's weight, being heavier than he thought & not much lighter than his aluminium bike.
Having said all of this, a good carbon frame will give much more than a weight saving, like stiffness, compliance & shock absorption.