I've seen this gamed out a few times. It results in a bloodier June/July battle in Normandy, Cherbourgs capture in delayed. The trade off is the German ground force is attrition away a bit faster, while the Allied losses are still replaced as needed. The result in all cases is the German position in Normandy collapsed a few weeks earlier.
In some of those games the Allied player saw the opportunity to seize a under defended port with a secondary landing, usually commandos & airborne. Then following up with corps delivered straight to the docks. Thus the German was faced with twin punches of this RANKIN type operation & the historical DRAGOON op.
How fast the Allied player advanced across France in August and September depended on how well he'd played in the June & July.