It's going to depend on the printer driver you are using. Since you are going to file you can probably pick pretty much any printer driver you want.
I did some testing and the HP Universal Printer Drivers (as an example) will go up to 600DPI when used with a file (they claim 1200DPI, but it's not actually available).
You can get the 32-bit version of the HP UPD here.
One of our B&W laser here is a Brother 1650DN, and it supports up to 1200 DPI. If I use its driver for a new printer that prints to FILE: I can pick 1200 DPI -- so there are drivers out there that'll do >300DPI, you just have to find one that works. :)
Also, ensure that whatever you're printing is in a resolution that high, or you're just printing a 300 DPI (or worse, screen DPI of like 72) at 1200DPI, so it'll look the same even with the higher output DPI.