If an f1.4 DSLR lens is stopped-down to f2.8 to improve quality, is this the same as using an f2.8 lens wide-open?

This will depend on how well-built your lenses are and the type of glass element used. In general though, faster lenses are usually built better. With that said, an f/1.4 lens stopped down to f/2.8 will have a higher quality image than a lens that is completely wide open at f/2.8 :)

In short, I would use the f/1.4 lens at f/2.8 instead of using the f/2.8 lens at f/2.8!