Why would you want to 'clean' a carpet using 500psi? You want to clean the fibre with as little moisture as possible not powerwash the damn thing 
Cleaning the carpet at 500 psi gives a much quicker and better clean. Doesn't make the carpet any wetter as strangely the more water you put down the more you can vac up.
Most truckmounters operate between 300 & 600.
Sorry, but I disagree. I've seen time and time again the amount of damage that is done by using high PSI. Take a look at a hessian backing, underlay & sub-floor once you've 'power washed' it with that sort of pressure. You're there to clean the fibre not the backing.
You say it doesn't make the carpet any weter?

Arh? There is a big difference blasting 500psi of water down to say 70 or 100 psi?

?
The amount of damage I've seen with these truck mounts and high psi machines

Before commenting, go and clean a carpet with that pressure, lift it back and just take a look....... you'll have a shock!!!!