IBM, in their infinite wisdom, may have picked heavier springs for a reason.
Hahahahahaha. Good one. Oh, I'm a former IBM-er...
I don't know one way or the other whether a lighter BS spring is possible. Would be neat, certainly.
I know from talking to Chuck at Unicomp that the spring alone is not enough. In a BS board, the springs need to be aligned a certain way on the hammers. If you compare any two hammers, and look at the end of the spring opposite the hammer, you'll see that the springs have exactly the same rotation. (This is not true if you look at the end of the spring adjacent to the hammer.)
I'm not sure what exactly Unicomp is controlling for when they assemble these things. I know that if you try to assemble springs and hammers by hand, you won't get the uniform key feel of a stock Model M.
Chuck also mentioned that Unicomp contracts with another company to align the springs and hammers. That's part of their secret sauce, it's surprising that they contract it out.