My son has one of Leigh's 15*, 66" under quilt and loves it. It works great on his Hennessy Ultralite zip. He's used it to 30* without any problems.
I was going to purchase a 30* quilt as soon as Leigh gets more in stock (though I can't decide if I want to try the 48" model or stick with the 66"). I also have a 30* top quilt on order.