Are you familiar with Skilak Lake and the conditions there?
You should be "okay" for leaves on the Kenai. I put that in parentheses because the leaves are so variable. A couple of weeks earlier is more of a sure thing, simply because there is less opportunity for wind to strip the leaves. We usually get our first big fall storms in this region around the fall equinox, which can do a good job of putting leaves on the ground. You'll still find good color in smaller areas and in more intimate landscapes, but the big strokes of color may be dimmed in large landscapes. You can also get good tundra on the hillsides above Anchorage, which you can drive to in short order on good roads.