I am quite familiar with the Signature package. I have a 2006 Signature Escalade. It turns out that that package was not a factory package. It was an after-market option added by some dealers. It is really a trim package that included rims (which may have been factory), emblems around the vehicle and inside. I was in an accident in which one emblam was ruined, and the others are in pretty bad shape. I have called dealers all around the country, body shops, and boutiques. I even found the website once that offered the emblems for sale and apparently offered the package to start with. I emailed them and have since been unable to find the website again. I did see that they also offered the "Vortec 6000" emblem also, which may have been part of the trim package, along with the pinstripe. Other than trim, "Signature" was not an item that affected options or power, or anything else.
I hope that helps.