Postscript is a programming language, bassed on Forth, so theoretically postscript files(programs) offer infinite fidelity, well as good as the rendering engine / interpreter that's passing the file can offer. There are numerous fractal / mandelbrot Postscript demos, that can demonstrate this.
As to whether there is noticeable difference between a 1200dpi bitmap generated from a postscript file, via one Postscript interpreter, that's possibly scaled later by another engine to fit some physical medium, and a natively rendered Postscript file on that device, it all depends on what the intended purpose / media is, and especially on how you will be manipulating the image (rotating / scaling).