1. Hops before boil This won't be an issue if the recipe calls for them to be a 45min+ addition.
2. Yeast pitch If the recipe calls for two, then use two. Most yeast packs are intended for 5 gallons of 1.040 wort. More yeast is needed for higher OGs. The LHBS may have been inclined to allow for more ester profile, since it's a Belgian Strong which relies on esters for much of its flavor. But again, I would follow the recipe with two packs.
Reduced nutrients This will hinder the yeast health, especially in extract brewing since it doesn't have as much nutrients as with All Grain.
With an underpitch of one pack you will have more esters, but run the risk of the yeast giving up before you reach terminal gravity.