You can post this in the business subforum to get more ideas on the finance side of things.
This sounds like it was an excellent use of your time. A letter from a well respected professor that talks about your contribution on a project that will have real contribution is incredibly valuable for admissions. I am wondering what you think could have been a better use of your time.
On top of that, doing research in this much detail should have helped you select schools more efficiently and it lets you know that you really do want a PhD. It also give you something interesting to put into your personal statement. Longer term having a publication in a top journal this early in your career is a great thing as well.
You should be set up well form this work. No need to worry.