Electric fuel pumps for EFI generate pressures on the order of 50 psi, the gas cap holds a pressure of 2-3 pounds. So at most its going to make a few percent difference in fuel pressure, and most likely not even that since the fuel pressure is usually regulated. So I'd say you'd never see a difference in performance at all, just the code for an evaporative emissions fault.
'66 Dodge Polara & '69 Dodge Coronet R/T both 440/727
'08 Ram 1500 4.7/545RFE
'12 Challenger SRT8 392/6-speed
'99 XJ 4x4 4.0/AX15, '14 WK2 4x4 3.6/8HP