I suspect that they will do all right as the majority of the camping gear shoppers will look no further than REI/EMS/ETC and will never know about the cottage industry offerings. Compared to what they have hanging off the rack, these bags/quilts will seem revolutionary.
As you may have guessed, I'm in the market and have been in a holding pattern to see what Thermarest had to offer. I'm sure you are right and the price at the store will be much lower which will blow my analysis out of the water.
I guess my initial reaction was "750 fill, Chinese, AND expensive?"