It's been done.
Not much of a performance increase, but it'll sound healthier. The mid pipe is also restrictive, and it's baffles can be removed, too, but it's usually not the easiest work on a used bike since the baffles get harder with age.
On a first generation (1G) B12, you can also replace the stock can with one from a RF900. These are less restrictive (and a little louder), but do nothing about the mid pipe issue.
The B12's will see a much bigger jump in power with jetting tweaks since the factory jetted them so lean... and if you add an aftermarket slip-on that comes with a new mid-pipe, you'll REALLY wake it up!