I agree with you that pressure is pressure but that was the problem with the small pancake compressor. I had the pressure set for the max (110 psi) and when the impact wrench was run continuous the air depleated from the tank causing the pressure to drop below 80 psi. The tiny motor for the compressor was not capable of replenishing the air quick enough. The motor was about the size of the air compressors that you plug into a 12VDC receptical.
I do not have a need to crank up the air pressure for my 6.5HP (correction it is not a 7.5HP) compressor, the compressor switch shuts off at 125 psi. I have three air taps for my compressor. Two are located after the pressure regulator and one located before the pressure regulator. I found a handy device at Menards for my air tools - it is a small oiler which I put quick air connections on it to use as an inline oiler. This way I do not contaminate my air hoses with oil and the air tools gets lubricated as I use them.
The 1/2" air impact wrench that I have requires 4CFM @90psi. In order to size an air compressor you would need to take the air tool which requires the most air consumption (CFM and PSI) and compare this to what the air compressor is capable of delivering. In my case my dual action sander requires the most air (6.5CFM @ 90psi).
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles