If the mappings correct there iwll be ZERO advantage with closed loop, indeed it may even use more fuel on closed loop.
Best thing I ever did was junk my L6, and went K3 Emerald, correctly mapped, pisses the mot emissions at 0.97% CO, 137PPM, with nearly 190,000 miles on it, and 40+ mpg on a run not trying esp hard for it, smoothness and driveability are in a different league compared with the previous mapped L6.
tabetha