I hate to be one of those negative naysayers, but imo he personally wouldn't have made that much difference to the fight, he probably had all the troops he could potentially have had at the time. But maybe he could have elicited more help from outside, and won the war with decisive action.
It would be very interesting in terms of Ireland, would he give them independence? What would then happen to the protestant settlers? Or would the Irish be happy to be part of a United Kingdom ruled by a Catholic dynasty? It seems crazy now, but remember that the Jacobites were fighting for the Stewart lineage to the British throne, not for Scottish and Irish independence.