How can I go from a CPL to an A320 Rating?
A U.S. pilot with a Commercial license can get a type rating by going through an appropriate course. It isn't cheap, but it can be done.

The benefit of having the type rating with only 300 hours is probably pretty slight, at least in the U.S.  Anybody hiring into the right seat of an A320 either has their own "ab initio" curriculum, or else they're probably looking for far more experience than that.  There may be a unique job out there that is looking for exactly this sort of applicant, but it seems hard to imagine.

You might be better off saving the money until you have a line on exactly what position you're after, and you're confident that they'd hire you with 300 hours (or whatever you have by then). Who knows -- you may find someplace that will be glad to hire you to fly something else, if you'll get the training on (whatever they fly) by going to get the type rating. Having the money available to spend on the needed type rating could be a lot more useful than have the 320 type at that point.