A member here summed it up quite well:
"The speed you know is the speed you have"
Or something like that
When I setup a bike its so I can ride that same speed all day long. Without setting up a bike I can still ride fast but long days get much longer when things aren't just right. There are certain places where a good bike makes a big difference but for most people its more about their ability than setup. Now once i've learned the techniques, setup helps a lot. Having good suspension always makes things much safer though.
Things like knowing when to hit the gas or when to unweight the bike makes all the difference in the world.
The thing that makes the biggest difference and covers the widest amount of situations is fresh tires. Right now i've got about 120 miles on a fresh set of M12's and the edge is already gone. I can feel it in turns where its really hard pack. Granted this is some very rough rocky terrain and i'm riding harder than ever but i'm still pretty surprised that I can tell a difference.