Graham,
It's all about water volume/lpm. Good Pressure is needed but lpm is more important.
I read somewhere on here when I started out and still think its the best to explain.
Imagine at a mini hitting a wall at 30mph (30mph being your 3000psi and the size of the mini Being you volume of water, say 12lpm)
Now image an artic lorry hitting the same wall at 20mph (20mph being you 2000psi and the lorry being your volume of water, 30lpm)
You know which is going to do by far the most damage.
Hope this help
Ian