I don’t know about the “average CFA charterholder”, but here’s my take based on GMAT topic areas and some example questions:
http://questionbank.4gmat.com/
Algebra, mean/median/mode/deviation, number systems, percentages, P&L, ratio/proportion/variance, simple and compound interest, are all in some way shape or form needed to pass the CFA curriculum, so I would assume most CFA charterholders would perform almost flawlessly on those sections.
Progressions, inequalities, geometry, measurements, probability, quadratic equations, set theory, speed/time/distance, and work are all sections that anybody smart enough to do the CFA should be able to master in a couple of hours each, IMO. Memorize a few things and you should be fine.
Data sufficiency is the only thing I could imagine any significant % of charterholders struggling with, because it has so much to do with the way your brain is wired to think.
Regardless, most charterholders I know would do extremely well at this stuff after an average amount of studying.