Looking at a 260 hp 5.9 Cummins in a super clean 2001 Expedition 34N that weighs 18400 dry. How much of a slug is this setup and will the transmission hold up to more power?
You'll never win a race to the top of a mountain, but as stated, it has plenty of power for the weight. Keep in mind, if your intention is to buy it and add power mods, it may be maxed out if it has the Allison 2500 and you'll be exceeding the Allison torque/HP rating.
On flat land you'll be running along with everyone else and maybe even getting better mileage than most.