Hefewiezens are traditionally highly carbonated, but they don't NEED to be for the beer to be good. Your beer's going to taste just fine with 2/3 cup dextrose for carbonating, it's just traditionally they are carbonated higher.
The reason NB likely kept the carbonation down in the kit is that the to-style carbonation of a traditional Hefe is right on the borderline of what some of the cheaper commercial beer bottles can handle before they start to blow. That's why most commercial examples of Hefeweizens are bottled in the thicker Belgian-type bottles.
NB wisely is not going to instruct you to bottle to 3.0-4.0 volumes CO2, which may lead to a few exploding bottles for the typical homebrewer that is using standard commercial beer bottles. There are certainly some liability concerns there.
All that said, I make alot of Hefes and routinely bottle with 8 oz (by weight) dextrose pushing 4.0 atmospheres of CO2 in standard beer bottles, and have never had a bottle bomb. Next time, go ahead an use just a bit more priming sugar for Hefes!