Keeping in mind the forces a power steering box can excert on a K-member, for it to be cast in aluminium, it would have to be beefed up in certain areas to prevent failure due to stressed mounting points.

Even when started out with a Borgeson sized box, I think because of the metal added for beefing up it would add enough weight that in the end one might only gain a couple of pounds over a castiron one.

Don't forget the pricetag either.
The 'new' Borgeson box is not cheap as it is. An aluminium version could easily retail for close to $800 to a grand.
At that point a rack-conversion has been starting to look very interesting already.

Why not go for some aluminium spindles while we're at it?