As a rule of thumb dry carbon is 70% lighter than steel, 40% lighter than aluminum, and 15% lighter than fiberglass. Wet laid carbon is more of a crapshoot but never as light as a dry carbon (but also nowhere near as expensive). This is why large parts are made from CF to save weight.
The mirror caps on this car are made from plastic. Carbon fiber (which if you want to break it down is essentially a fiber suspended in epoxy, a plastic) will not save weight over plastic. Will it be stronger? Sure. Does strength matter for that part? Of course not.
It's a strictly visual upgrade. If you really are worried about weight, leave the plastic caps alone.
Having said all of that.
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