Discuss GMAT score in optional essay with 44Q (57th %ile), 7

Hi,
Absolutely. I would say that you should explain not only the low GMAT Quant score but also anything that you would want the admissions committee to know of, which did not necessarily come out in your application.
In order to make the additional essay not look too much of a laundry list, you could weave in a story or a way to connect the things you are mentioning in the essay. If it reads well and doesn't stand out, the admissions team wouldn't mind at all.
Often students try to over analyse the purpose of a section in the application, i.e. additional essay. It may be best to write down your side of the story and let the admissions team decide for themselves.
I would also ask you to think a bit about this dichotomy - you are clearly quite quants focussed but yet your score doesn't reflect so. Rather than mentioning that you have built models and major in quant focussed coursework, I would urge you to think why your score doesn't reflect your abilities (clearly it doesn't) and address that - i.e. maybe a section or a particular question type on Quants - why is that happening and how can you demonstrate that weakness isn't necessarily relevant to you. The more soul searching you do now, the easier it is to explain this.
Hope this helps.