I'm not an admissions person but 800 math/520 verbal is really good, and you probably don't need to retake it for your verbal score. Most admissions sites for FE programs only post the desired math score and, regardless, 520 is a pretty solid verbal score anyway. Besides, as a quant you'd be expected to come up with pricing models or trading strategies, not quote Shakespeare. And you wouldn't want to risk getting less than 800 on math if you retake it...I don't think you can mix and match your best score on each section.