Deriving the B channel on a normal map

I found this function in the VectorOps that I'd never noticed before. It's really cool. My plan for normal maps this semester was to just use the R and G channels and append 1 into that. (top row) With this function (middle row) I can still pack my textures but I will also be able to get an accurate B channel. Obviously appending 1 would be cheaper but it'll be interesting to see if there's a noticeable difference in appearance that's worth the cost. Bottom row shows how the function works. √(1-(xx+yy))