Kramer horsehide belt scabbard
I've tried a number of different holsters for my Colt Defender, a similar gun to your Kimber Ultra. The one that conceals the best for me is a Kramer horsehide belt scabbard, an outside the waistband holster made of very thin and stiff horsehide. It positions the gun high on the belt and pulls the gun close. With a good cover garment it conceals perfectly.
Shoulder and IWB holsters
I have never used a shoulder holster and don't know much about them. You mentioned IWB holsters, however. I do use those with guns that are small and slim through the slide or barrel, such as a Kahr MK9 that I have. One IWB holster that I like is the Comp-Tac 2 O'clock model for appendix carry. This is made of thin and strong Kydex material that doesn't add much thickness but holds the gun firmly. The gun "clicks" a little when it goes into or out of the holster. The appendix position is about the fastest spot to draw from, and allows an easy draw when seated, which is difficult with behind the hip belt holsters. But appendix position only works with smaller guns.