I don't have a 38K but I have had an 08 Expedition 36D and currently have a 2018 Discovery 40D. The Expedition had the 6.7L motor and the 40D has the bigger 8.9L motor. The 6.7L is a decent motor, it will keep up with a V10 gasser that weighs 10,000 lbs. less. When driving the 6.7L you have to keep an eye on the terrain, if you see a hill coming, you want to get on the "go" pedal to build up a little speed. It gets pretty reasonable fuel economy my old 08 averaged 8.5 mpg.
The 8.9L engine is worth the cost of the upgrade in my opinion. In most cases, the cruise control will easily maintain its set speed. It takes a pretty big hill to make the transmission down shift. As my motor has broken in, the fuel economy has risen to 8 mpg. YRMV
Steve & Margo
2018 Fleetwood Discovery 40D LXE "Alice"