There is no way to withdraw a pistol from a concealed horizontal shoulder rig without sweeping the arm.The offhand has to get the cover garment out of the way-which means the pistol has to cross that arm on the draw. The problem is worsened the lower the shoulder holster is worn.
Putting the gun in position Sul would seem to worsen the time problem . Ive timed myself drawing shoulder versus waistband carry, and I ditched it after realizing the Miami Classic II added a full second and a half to my FAST times.The additional complication is reloads-reaching across the chest for a magazine takes more time for me then the belt, and clasps on the magazines means you wont be getting a FAST coin with Don Johnson's holster.
Addendum-long barreled handguns like a G-17 are out unless you have a big chest front to back, or your holster cants the gun upward like the old school Jackass rig.