Been a busy day, so first time back. I like the horizontal layout also. May I suggest instead of the blocks being 3 up, 2 down, it just be 5 across? That would free up a few inches of space in each post, if you also lowered the size of peoples profile pictures.
You could also take the thumbs up./down/checkmark/etc buttons, and place them between quote and report. Free up another inch or so. Them are the only things I see that are "Wasting space". I don't believe they are a waste of space, I just think the addon developers built them without considering screen real estate. Of course my suggestions might make everything more crowded/bloated, and worst than the current layout...
An awesome MSpaint that shows an example. It doesnt look like much, but when you multiply it by xx threads on a page, it is quite a bit!