Just an off the cuff reply, but porcelain in ca. 1830-1860 contexts here in
the Midwest region of the U.S. is a relatively rare phenomenon, at least in
my experience.  Way less than 1% ...  Frankly, your definition of
blue-sprigged soft-paste porcelain does not ring any bells for me.  The only
blue-painted wares I'm routinely familiar with are the pre-1830 floral
earthenwares.  If somebody can cite an accessible illustration of what you're
describing, that would be helpful.
 
However, I would add a second thought.  Despite the dominance of the British
ceramic industry throughout the Western and Western-dominated world during
the nineteenth century, I think that you are going to find some distinct
marketing differences throughout the affected area.  Sorry, I don't have my
library in front of me, but I have noted over the years that Canadian
descriptions (in the archaeological literature) of mid-nineteenth century
transferprints appear quite different from the contemporary transferprints
that are ubiquitous in my region, i.e., different manufacturing centers with
a distinctively different looking product.
 
Any body else ...
 
Mark C. Branstner
Great Lakes Research Associates, Inc.