quietman, I've been reading your posts here for the past year or so, and I'm registering an account for the first time to give you some advice that I hope you will heed. If you have your heart set on HKS - and it very much seems you do - you will need to retake the GRE and get a higher quant score. You will not be admitted with a 155 quant score, full stop. I know this runs contrary to much of the advice you have gotten above, and may even seem mean-spirited and deliberately disheartening, but I'm telling you the truth with the hope you'll save yourself a lot of trouble down the line, and furthermore, I am in a position to know the veracity of my statements.
Here's a dirty little secret about admissions at the very top programs, e.g. WWS and HKS: they toss most applications with sub-70% GRE quant scores unless the applicant has a major offset (minority applicant, incredible life story, CEO of their own successful company, military, etc.). They look askance on applications in the 70%-75% percentiles, as well. Above 75% you're probably fine, and above 80% they're looking at other parts of your application. If you really want to go to HKS, you'll want to pull at least a 160 on your GRE retake.
Now, if you call the admissions office at HKS right now and ask them, they'll tell you they take a holistic approach and that no one factor is responsible for an applicant being admitted or denied. Hogwash. Quant scores are a huge factor in rejections, and rightfully so: they are an easily readable metric that all aspiring applicants present, and they correlate relatively well with future success in grad school and down the road. Low quant is an application killer, doubly so if you're just a run-of-the-mill candidate with a high GPA/verbal GRE combo, of which there are many.
I should note here that it wasn't always this way, that five years ago you may have been able to swing this. But the times have changed. HKS is more selective than ever, and they're not rolling out the red carpet for sub-70% quants.
Your strategy of taking supplemental quant coursework, while not terrible and probably a good idea on its own merit, will not overcome your 155 Q. You will be rejected again by HKS, and will have wasted two years of your life trying to get into Harvard. Instead of spending thousands on statistics and econ classes, you would be infinitely better served by bumping your GRE score five points. Until then, that 155 is the kiss of death to your Harvard dreams.
That's the bad news. The good news, and you should really take this to heart, is that you can easily get a 160+ on your GRE. Seriously, it's not that difficult, especially for someone capable of getting a 99% score on the much harder LSAT. Five measly points comprised of high school level geometry and algebra and some number theory questions are all that stand between you and a successful HKS application (assuming the rest of your app is up to snuff). I find it almost inconceivable someone who could get admitted to HLS can't get into HKS if they put their mind to it; I am completely certain of your ability to get into HKS if you're willing to put the time into the GRE like you did the LSAT. Your problem is you've psyched yourself into believing you can't crack the GRE quant, when in reality you could have probably already retaken and gotten a 160+ by now. Logic games are harder than 99% of all GRE quant questions, and you did pretty well on those, yes?
I'm not writing all this to discourage, merely to tell you how things are. I'd hate to see you waste yet another year taking supplemental courses, only to be crushed when you get rejected again. You're completely capable of going to HKS, but it's not happening with a 155 quant score on your GRE, no matter how many As in stats/econ/calc courses you stack up in the mean time. You have to believe in yourself, and should be very encouraged by your LSAT score.
Who am I? How do I know all this? I have a personal relationship with a member of the HKS admissions council. Everything I've written above came directly from the horse's mouth, inasmuch as the horse can speak unofficially. I've seen how the sausage is made at these elite schools, and I simply can't stand idly by and watch you waste your considerable talents on a fool's errand. Take the time, do it right, get the 160+ Q on your GRE, and enjoy your time at HKS.
Final thought: you don't have to believe me. I could just be some random guy making stuff up on the Internet. Maybe you will be the extreme exception to the rule, and get admitted to HKS with a sub-70% quant score. The odds of this, however, are not in your favor. I sincerely hope you do believe me and take this message to heart. You're totally capable of attending HKS, but you'll need a better score.
Good luck.