For moving air like ordinary fan, don't think there are enough strong points to warrant the replacement of current plastic impeller, the main reason is obvious, balancing the carbon fiber blades will be a lot harder compared to injection molding technique at plastic blades/impeller.
With current matured plastic injection technology, an evenly distributed mass and seamless spreaded material is the key for a good impeller that will make a good fan.
No matter how advanced the bearing technology is used, it will be useless if the impeller itself is difficult to balance.
The only technical advantage that I could think of using carbon fiber impeller is only lighter blades (not very sure though), hence lesser power consumption, but I don't think fan power consumption is a major issue right now, don't you agree ?
While the stiffer or tougher nature of carbon fibre over plastic is not an advantage point for ordinary computer fan blade cause they're just slicing air. The only possibilities are only on big fan that can be as big as the pc case, cause these big fan needs a stiff material.
While for smoother surface, fiber construction is not very convincing vs plastic, plastic can be shaped/made into very smooth glass like surface easily to minimize drag.
I know you mentioned to exclude the cost discussion, now, just try to imagine the 'extra' cost in manufacturing the carbon fiber vs plastic impeller, its just doesn't add up and doesn't make any sense from economic point of view.
Maybe it will be useful for ship/boat impeller, definitely the impeller will sustained heavier stress in the water, but that is probably at another argument/debate against the cheap and light "metal" impeller.
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