Depends on....

Density of glas fibre (s-type) is about 2.5 kg/m3
Carbon is about 1.8 kg/m3
In hand made laminats you have about 30% to 40% fibres the rest is resin. Hence you save a couple of kg.
The boat would be more stiff and has more strength.
Designing the the boat to same strength (whatever that is) would lead to about 20% to 0% of weight saving (in the hulls without fittings)
Basically evertime where your structure is driven by pure strength you save weight, when your structure is driven by minimum skin thickness or local damage tolereance etc. weight saving goes to zero.
If the part is strength-driven, you would reduce skin thickness, but less thick skins are more prone to penetration and buckling, etc. so you can't use the full benefit.

Yes, marketing people will tell us about big benefits, but check reality
* B787 is over weight and not up to strength
* Mitsubishi announces to make the wing of their futur regional jet out of aluminium, because it is lighter!

By the way, the strength (not stiffness) of a beach cat hull would be improved by factor 1.5 ... 2 with epoxy resin, instead of polyester.
Another efficient way of weight saving would be efficient structural design and quality control during manufacturing, both are less expensive than CFRP. ... but CFRP looks more pimped.

Cheers,

Klaus