Cfm delivered @ *** psi is the output. You can have 1000 gallons if air storage but if the tool uses more than the compressor can put out, sooner or later you will run out.
My thought is if tank volume is a variable in CFM calculation, introducing the additional volume with the same size motor can reduce realized CFM at pressure "X" when using the tool.... ???++
Or... I'm not fully understanding your configuration Chris.