Seems like you have covered the essentials here. I can't think of anything else.
Since the Qiagen patent on spin prep columns ran out, these kits are very cheap - $0.40 each? In the 3 or 4 kits Ive used, they all seem to use the same protocol and about the same buffers, so there might be differences in quality or yield but if so, they are small.
You can even make your own buffers as the recipes are on open wet ware. None of the kits i've used has suggested that these buffers can't be used. They even usually call the buffers by the same names.
So your application would have be pretty discriminating to differentiate. Even if this were the case, the quality could probably be improved a lot just by washing the column more. Can you provide more details about how a few percent one way or another might be an issue?