First of all it would mean a change of the armor tactics, in OTL it was mobility before armor, so you would have to change that.
A better idea instead of an early pershing would be if the M6 heavy tank design would have been succesfully put in production, so instead of the sherman, there would be an additional tank.
The most reasonable thing to do would of course have been taking up the offer of the british, and fit much more shermans with 17 pounders.
the gun was available in late 1943, so if it would have been fitted to say 50% of the shermans, this would have made a big difference. (not 100% because it seems the the 17lb gun was very good at anti tank, but not so good at gp (He shells)