I wasn't accounting for it, but I assumed it wouldn't be significant. The efficiencies claimed are over 2x better than chemical rockets. You don't get that much just from changing gasses.
Yes, you can. Hydrogen, for the same temperature, has a far higher speed of sound (which is close to the average speed of the gas molecules) than air or water vapor. This is why your voice is higher pitched when you breathe in helium (also a light gas like hydrogen).
Are you accounting for the fact that the NTR exhaust (hydrogen) is lighter than chemical rocket exhaust?
At the same temperature, both propellants have the same average kinetic energy per molecule, so the hydrogen must be moving faster.