vista isn't much slower at all, dx9 in vista, at least on ati drivers

p ) is pretty much as fast as XP, nvidia takes a much bigger hit from their, honestly, not great vista drivers, but still not big enough to really be noticed.
as for framerate, its a freaking rts, 30fps is more than fine, 20fps in most situations and you'd be hard pushed to tell the difference. the benchmark stuff is basically never ever replicated in games, you just don't see fast moving units, choppers move slowly and most planes you barely see due to view in game.
as for dx10 vs dx9, yes, zoomed out i think theres very little difference. i did turn to dx9 to play the game as i ran the benchy first and thought it might not run great. i gotta test it, but i was pretty sure that close in, textures on the tank in the benchmark were much worse in dx9. also though faces, zoomed in to see soldiers, and other textures in general look much nicer close up. but thats fairly unimportant due to the fact that except that 1 minute at the beginning checking how nice stuff looks zoomed in, you remain zoomed as far out as possible for every other moment of the game.
but yes, framerate jumps up a lot in vista when turning off dx10.
EDIT:- actually when i opened the last two comparison shots in new tabs, there are LOADS of differences. they might not massively contribute, but theres a crapload of extra stuff. every single tree leaf is much more pronounced, rather than a kind of singular blur they look more defined and individual. most of the houses have extra detail on the windows, further back you go the better they look, again not massive differences, but they are there. almost every crater's effects stretch out further and look slightly better for it. there are shadows under the small wing bits on the choppers where the guns are mounted, so each individual unit is being dynamicall lighted/shaded, on dx9 the whole unit is lighted the same, and under the wings are the same brightness. You can also see a big difference in the lighting, its more realistic in dx10, where as everything is just , overbright in dx9 its better coloured in dx10, also with better shadows behind buildings at close and further distance.
then another review, forget which one, seems to show that dynamic lighting is done on a much bigger scale on dx10, with clouds over head and beams of light being shown over the current battlefield, where as in dx10 the lighting is uniform no matter the clouds, and it only adds the "beam" effects in the far distance which don't change(same as dx10 there).