I make my wand passes at the same speed on virtually every job. Average pressure at 300psi, up to 400 for real grot jobs on cut pile, and down to as little as 200psi on tuffed low profiles.
Reason I drop the psi for tuffed low profiles, is because if you hit them with 400 psi, no matter how swiftly you move the wand, the psi will blast it through the carpet wetting the backing and at least the top of the underlay.
Dave.