Seems that this file has the potential to get very big, won't this be bad for performance in the long run? I feel this will bloat to hundreds of MB in size on a populated monthly server. 

Well, theoretically if you're hosting couple thousands of players, sure it could get to like a gig. 

Realistically, it doesn't store enough information to get to that size. Sadly I didn't have the capability to test it out to that extent, if however you do get to that point and the file seems to get rather large, I would appreciate you sending that file over to me so I can optimize it better

Even if it did become a problem, you could probably make the data file use SQLite which would resolve most performance problems I think.... as then it would only rewrite one line at a time.