See this as to why I think you won't be wasting your gmat prep: what-is-the-best-way-to-start-the-gmat-journey-98067.html#p754966
Also, from what I understand (have not take CFA), is that the two don't have much in common or at least people who have taken CFA did not do well on the gmat... I think it may provide a sense of security around quant, which is not accurate.
Finally, see this about CFA: search-results.xhtml?cx=009332468639467955845%3Ah-z9cduzcoi&cof=FORID%3A10&ie=UTF-8&q=cfa&sa=Search#1215
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