Hmmm, I'm not following. If the Greasemonkey script just leaves you with the resulting text content (column) that will fit to width, then I can't see how portrait would let you fit and "more" text on a screen then landscape. If anything landscape should fit the same (or possibly more!) content.

Given the same font size and vertical line spacing, portrait would have more (albeit sorter) lines of text, giving it additional opportunities for wasted space due to word wrap. No?