There are two specifications you want to know - and neither one is horsepower.
An impact wrench will be rated for xxx pound-feet at an air supply of yyy psi @ z.z CFM. If your compressor cannot supply those figures - or supplies yyy psi at fewer CFM, or supplies z.z CFM at fewer psi, you won't develop full specified torque. I am unsure of the relationship, but I'm sure it can be calculated.
5-90