Corrylan,
I don't think you are missing anything. The ICH7R south bridge feeds the Silicon Image Sil4723 chip which performs all functions in hardware to deliver the data to both hard disks plus some nice maintenance and error correction as well. Since the Sil4723 gets the full benefit of a full bandwidth connection (3Gb/sec) to feed it, the Sil chip should be able to feed both attached hard drives (EZ-RAID) without compromising performance. Now, having said that, the Sil chip does add another layer that must be traversed to get to the disk drives but I hardly think that would make a noticable difference given the max sustanined data rates of hard disk drives.
The very valid point you state is the max sustained speed of disk drives on the market today means that there should be no problem. Drives can't even meet SATA 1 speed limits much less SATA 2 in actual use. I have seen some claimed cases, like the one I repeated above, that show differences in burst rates but these are not sustained rates as you correctly pointed out. The later is what one MIGHT be able to detect in actual use if you are dealing with huge file sizes all the time.
In my own system, I have a SATA 1 RAID 1 data set. I used SATA 1 drives because I had installed them only 6 months previously on my old system which only supported SATA 1 spec. When I went to build new system I looked at a lot of data and claims and decided I didn't need SATA 2 spec drives because I would never see any difference in performance. The new Raptor drive I bought for the opsys can only be gotten in SATA 1 spec.
My new system is lightning fast and I am very pleased with my decision. I think SATA 2 is a marketing gimic because the hardware in real use can't perform to the level the electronics are capable of performing. The main benefit of SATA, IMHO, is the cost and size, safety monitoring, and data write optimization available versus traditional IDE disk drives.
Paul