I can't speak to the PRS-300, but I'm very impressed with how well the PRS-600 handles non-image based PDFs. I reckon the 300 does a good job as well. And if it does as well as my Pocket Pro (with a similar screen size) I think it would be more than up to the task.
The only reason I ever go through the trouble of converting PDFs to EPUB or whatever is when they've been encoded in some non-standard way such that the fonts are too exotic to read comfortably, or where line-breaks occur in the middle of words, like right before a final "s" etc. Then they're worth converting.
When the original PDF has really busy footers or headers, it can be tricky to remove these in Calibre, unless you've managed to pierce the veil on how it employs regex. I haven't quite gotten there yet, so I'll do an intermediate conversion to HTML, then use regex in a text editor like EditPad to strip out as much of the extraneous header/footer/page numbering code that came over from the PDF, then use Calibre to convert the HTML to EPUB. This has worked really well the few times I've needed to do it.
But for the most part, I just deal with the little bits of weirdness in the original PDF by ignoring it.
Sometimes I think it would be fun to get a huge magnifier like the ones on those CRT screens in the movie
Brazil. Then I could read PDFs in portrait at small size, where the entire page displays, and have it blown up to full size by the retro magnifier. Might fetch me some odd looks at my favorite cafe, though.
Good luck!
p.s. killdanzig: love the Firefly ref, m8!