If the Japanese force had been sunk at Savo Island

How would the campaign change if the American fleet avoided the night ambush and sank the Japanese force at Savo Island?

The campaign would probably end sooner because Japan's already weak supply position would deteriorate faster.

Removing the Japanese force at Savo Island would probably shorten the campaign considerably. Japan already struggled to deliver food, troops, and ammunition to Guadalcanal. Fewer ships would be available for the improvised runs on which the garrison depended. Logistics shape the outcome more than the surface action alone.