If there's ever a time to run a Banshee, it's when your only heavy choices are a Dragon or Quickdraw. This will be one of the very few times that the Banshee will be an upgrade, and if you're still facing down mostly mediums with the very occassional lighter heavy (and you probably are if you've not done Smithon yet), then the relatively poor free tonnage won't be quite so apparant and the slight speed boost means it'll keep up with the slow mediums like your Centurions, Hunchies, Enforcers, Hatchetmen and the like.
For 4 million, I'd say it'd be a rather solid investment to be able to bully stuff this early. Plus you'll be able to leg almost anything you look at if you don't core it, so it'll make getting another upgrade even easier too.