please refer to the many posts regarding general UI size that have been posted in the last few months. in particular, h9 still is very space inefficient compared to h8.
This is pretty much what I did when I designed the Compact mode. In some areas, yes, h9 is still lagging behind, simply because there were many other things needing my attention. Compact was designed to make H9 use the same (or less) space than H8. One of the spots I didn't get to was thinning down the input fields and menu buttons, which was on the todo, so as a result parm dialogs use a bit more space in H9 compact than H8.
However, in general, the H9 main interface in Compact mode is almost identical to a corresponding H8 main interface (run both at 100% scale, maximized, and turn off the shelf in H9 because it's an obvious addition to the interface, and flip between the two). What I mean by this is that the toolbars, pane headers, stowbars, spliters, etc.. are all very close to their H8 size.
i think you misunderstand how people like to layout their space when they have a high rez screen.
Some people, yes. Others don't like to go blind

ok, rant off. getting back to the issue - here's some specifics:
(these all refer to H9 Compact UI mode):
- Compact reduces the split bar & stowbar width by 25%, making it much closer to H8. Any smaller and the > arrows can't be drawn effectively. Anyways, H9's stowbars are 1 pixel wider in most cases (the size for stowbars wasn't consistent in 8, so your mileage may vary)
- The playbar in Compact mode is exactly the same height as in H8. The range slider is taller, however, in H8 there are a couple of pixels of wasted space beneath it. Horizontal spacing could use a bit of trimming in Compact, I agree. I think what you may be referring to here wrt vertical space is the status bar, which is a bit larger than H8's (and would get smaller if menu buttons were trimmed slightly).
- The 3D viewport toolbars are comparable in width to 8. The icons are larger but extend all the way to the edge of the bar, unlike H8, which is padded. This makes a small difference in the overall vertical size (though h8 again has more spacing between the icons).
- The pane headers are actually smaller in 9 (you can also stow various bits of it as well through the (v) button, “Pane Interface”).
- Horizontally, the menu buttons in H8 aren't much smaller than the ones in H9 (see the Rx/Ry/Rz menu in a geo object). However, the H9 font is larger, so the longer the label, the greater the difference.
- In H9, you can move the main menu, shelf, and status bar to any pane, much like you can with the playbar in H8 (this is part of the desktop). So you can reclaim a decent amount of space this way as well (pane menu, Pane Interface, “Move ___ Here”)
I'm not trying to be contrary, just pointing out that many things have been worked on in direct response to user's requests in this area. I'll see what I can do to handle some of the remaining hotspots. The network editor is a whole different ball of wax, though.
Edit: Note that Matt's image above was taken in Normal UI size. In Compact, the gear icons are smaller and have a negligible impact. “Compact” is really what users desiring more UI space should use, not UI scale.