Will new XHD springs relax a bit?

All springs settle over time. ALL. Doesn’t matter how “good” they are, over time they will sag with use.

That being said, how much they sag and how long it takes depends on the properties of the metal used and the manufacturing process. The Mopar Performance made XHD springs I put on my Challenger sagged dramatically over the course of only about 6 months, they lost close to 2” of ride height. It went from sitting considerably higher than I wanted it to actually sitting lower than I wanted it. The AFCO springs I have on my Duster have only sagged about a 1/4” in the 5 or so years they’ve been on the car. I probably wouldn’t have noticed it at all if it wasn’t for all the frequent ride height measurements based around some of the suspension work I’ve done.

The Mancini springs have a pretty good reputation, and based on what other members have said here I wouldn’t expect them to sag a lot. So if you car is sitting more than a 1/2” higher car than you want it you’ll probably have to alter the ride height with the spring eye position or lowering blocks.