terrastrife, I going to have to refute your claims. My array has a final parity check speed of 75 MB/s and I am using 3 WD Green 2TB drives. They start off at over 100 MB/sec. That puts the difference in speed from 95 MB/s to 75 MB/s a drop of about 22% {the math: (1 - (75/95)) * 100 }, which is nowhere close to your claimed 50% performance loss.
Perhaps your drop in performance is from the adding of another drive, regardless of it's type.