With my 2540 PSI power washer, hooked up to a 75 foot garden hose, I could have sprayed a 40 foot tall tree with no problem just by adjusting or changing the the nozzle.
I have it more than 5 years and I can assure you that it has not been the victim of too much tender loving care. My brother in law used it for more than 5 years in his business and I have it for 5 plus years. That Honda engine has not had any service since I own it and I am sure it is not putting out as much as it could if it got tuned up.
I think that the biggest problem you would have with a 300 foot hose would be moving the hose.
Mike