First off, I was gonna say that you should probably flush the fluid completely, but you probably did that when you changed the rack, so I'll rule that out.
When you're running a a tire wider than stock, it is only natural to put more stress on the power steering pump. Actually, it's not really the width that puts more stress, but rather the scrub radius. Your wheel rotates around the same axis no matter what offset of width you have on the wheel, because it's attached to the spindle that only moves around the top mount and the ball joint. The easiest way to turn the wheel with out moving it, is when you turn it exactly on it's center. So if the center of your wheel is on the same axis as the steering axis, your scrub radius is in the center of the tire. This causes the least stress on the steering system (pretty much everything from the steering wheel to the ball joint), and the wheel will turn easier. When you offset a wheel to the outside, which is probably the case with your cobra wheels (I know my Cobra R wheels did that) you move the scrub radius to the side, and causing more scrub to happen when the wheel is turned. This puts extra load on the pump and other steering components.
To check your scrub radius, all you have to do is turn your wheel lock to lock several times, and then roll the car back a little, so you could see the scrub circle that formed on the tire. If the center of the circle is not at the cetner of the tire, than your scrub radius is not centered and this means that the steering is taking extra beating. From my experience, factory 17x8 GT wheels have centered scrub radius, while Cobra R 17x9 wheel, the scrub radius is moved inside about an inch. My pump is a bit loud too, but it's nothing bad really.
