I cannot comment much on additional fonts.
There is no straight forward way to solve the problem.
You are absolutely correct about embedded fonts. It incredibly slows down page turning.
Since you are going to use a hacked firmware to be able read korean, maybe it is possible to modify the firmware and add additional fonts before you upgrade the reader.
When I just got prs-500 I had this notion in my hdea that I would like to use Verdana or a similar font because it was designed specifically for reading documents on comupter screens and it is easier on your eyes etc. Unfortunatelly, sony reader supprts only 2 types of fonts natively, one of them is Swiss and it remotely resembles Verdana but after a few months of preparing all my books always in two copies, one with Times New Roman and another with Swiss, I have decided that TNR is absolutely fine and acceptable font and since then I don't bother anymore and just read books
