Congratulations on the fantastic performance, justnorth! Like other members who have replied on this thread, I do have some concerns about a retake. But I will directly respond to your question first.
I would not advise taking a prep course to increase your math score two points. It's definitely not worth the time or money. Besides, Knewton probably isn't designed to raise your score to that level (as souvik101990 mentioned). A 49Q suggests that you can obtain a perfect score on your own. So the
GMATClub Tests are highly recommended. Also, look into Jeff Sackmann's Extreme Math set. Try to find any weaknesses in your math (geometry, timing issues, etc.) and focus on those. You will have to be extra observant here, since your weak points will be more subtle and harder to detect than for someone who scores a 36Q.
As for the retake itself, I am against it. Perhaps you can spin it and say that while a 750 is great, it doesn't truly represent your potential. One could counter that retaking is not only risky (your score can go down or not change), but it also shows your inability to let go. You can rebut this, but your argument for a retake so far is this:
justnorth
The quant percentile (83%) is on the low side for those MBA recruiters that look at GMAT scores, even though the overall 750 should be more than sufficient.
I'm not sure that you fully appreciate the scores and their relationship to the corresponding percentiles. A 49Q is a 49Q, which is amazing. In fact, it's so amazing that it's almost indistinguishable from a 51Q. You're putting too much emphasis on the quant percentile, which the admissions office doesn't really pay attention to. The emphasis should be squarely on your performance, which is most reflected in the actual score - not the percentile. And what about verbal? You mention the Powerscore CR Bible (an excellent guidebook, by the way), but it seems that most of your attention is on math. If you truly want to improve you performance, then the majority of that should probably come from verbal, since that section has the most room for improvement.
My only other concerns are similar to those of gurpreetsingh. Your strategy could be a turn-off to some adcoms and take away from other equally important aspects of your application. Essays and recommendation letters play an equally important role in the application. But I have already taken up enough of everyone's time. I don't mean to tear down your ambitions. Just make sure that you're doing this for the right reasons and that you feel good about it. Whatever you decide, I wish you the best.