The power steering cuts out at high RPM. The idea was that you'd have plenty of power assist in parking lots, but more direct feel when you keep the RPM high. There are some modifications people have done, especially for autocross, where the speeds are low, and steering effort is high without the assist. You basically shim the cutout valve inside the PS pump so that some power assist is retained all of the time.
Steering that gets light and floaty at high speeds even with the revs up is partly a function of tires. Unless you modify the rack, add a steering quickener or put a softer spring in the PS pump relief valve, the only other thing you can do is either switch to narrower tires or a less sticky compound (or run shaved tires with less tread depth). The softer the tire compound, the more vague the steering will feel - often a major complaint of people running R-compounds for the first time.
One last thing you can do to make the steering lighter and more responsive is to dial in a little toe-out. It will make it easier to turn the car, but it will make the car less stable at higher speeds (unless you add caster)
