All of these questions can really be answered only by knowing what it is
you're trying to accomplish. The "how much difference" question is hard to
say, unless you're looking for a technical figure. I can tell you that
unless you're building an autocross or full race car, the "uprated" valves
are probably not the way to go. Too stiff. I can't make an accurate
comparison because I have SPAX, but I've experimented quite a bit with those
and have resolved that for daily driving the best setting is the softest
one, even with performance issues in mind. If I were to Autocross, I'd
tighten them down a bit.
A 3/4 sway bar is probably a good idea if you just want to improve the
handling a bit. Remember, the thicker the sway bar, the less traction that
end of the car gets. So, if you're experiencing understeer now, don't get a
bigger bar. I have a 7/8 sway bar on my car now, and IMHO it's too thick
for daily driving. The ride is edgy, and there's a bit too much understeer.
I plan to switch to a 3/4.
Hope this helps,