No issues with a 1.5 nut on a Signature Standard or Deluxe. In fact, we do it all the time on the Stanley Clarke models now, and it's not uncommon as a custom request on other models.
Here's a cone peghead on a 1.5 Stanley:
We tend to place the strings sorta close to the edge of the fingerboard for some people. The usually edge allowance(the space between the edge of the string and the edge of the fingerboard) is .15 inches. There's a bigger edge allowance on a Fender Jazz bass, so the strings are actually farther apart on ours, even though the overall width matches. Make sense?
It's no charge to make it anything you want, you just have to specify it of course.