That option isn't available. But the difference is dots that are 3.3
thousandths of an inch vs. 1.67 thousandths! And the file size will be
roughly 4 times as large.
Whoops, I misspoke, you can can edit the entry in the registry, to
output 600, BUT the interface/printer setting will still read 300. Also
if you play with the setting in Printer preferences it will reset to
300. The key in the registry is