If you would honestly read the message traffic for both sides, you would realize that yes, it would have made a very big difference. There would have been no attack on Sumter at all but for the arrival of those ships.
See Beauregard's message to Anderson asking him to refrain from firing upon them if they got into an engagement with the ships.
It would have made no difference. Davis would have used force to prevent any resupply effort. All he had to do was prevent resupply for a few days and Sumter would have surrendered. If he allowed to be resupplied, he would have a political firestorm on his hands.