You would prefer to double the thickness of the Air so a traditional SSD would fit? Or perhaps lower battery life to free up some space?
You're thinking of desktop SSDs. mSATA SSDs are roughly the same thickness as the SSD in the Air, and actually have a smaller footprint (about 60% of the size of the Air SSD -- slightly wider and half as long).
And you can get a 256GB model for about $200, versus $320 for the custom OWC SSD.