Under armour 4.0 bottom layer cut to capri length is warm enough for me under my speed suit when it is -20F.
I dig the UA 4.0 stuff as well, quite warm. I have an old pair of fleece expedition weight Duofold long undies that are fitted with no real bulk that I cut to 3/4 length that I stuff under the speed suit as well. The thick fleece is nice for the chair ride, much more insulation.
Skiing in Michigan is always cold. Shell pants can't keep up unless it's a bluebird day, we simply don't have the vertical to stay warm. Insulated ski pants are a must. The 3/4 length cut fleece pants are a huge boost. When it's really windy and cold, a merino union suit makes for a toasty day.