The pressure and volume of flow that a pump can provide are inversely proportional, to a large extent. So, my question is; how can one know what the pressure limit of a pump is?

For example, everyone seems to believe the Shurflo pumps (~1.8gpm@45psi) are great for water injection, but the average WI nozzle flows around .06-.08gpm @ some unknown psi (probably around 60-100). This flow/pressure, of the nozzle, is significantly lower what the pump is rated at, so an inferior pump would seem to be more than adequate. Basically, how much of a cheap-*** can I be?