Adjusted cooking time for an already-baked pie shell
Most pie recipes that begin with a high-temperature setting and then lower it are to bake the crust.  It isn't necessarily for frozen crusts only, but also for fresh-baked crusts that often need high heat to set and create the "flaky" pastry texture that many people desire.  (Also, unbaked crusts can absorb too much liquid on the bottom before they set without this high temperature step.)

If you're using a pre-baked pie shell, you're correct that this high-temperature step likely isn't necessary.  In fact, it's probably better not to use a high-temperature step on a pre-baked shell unless a recipe calls for it, as excessive crust browning is a problem with many pies that bake a long time to begin with.  (Which is why people often use foil or pie covers toward the end of baking.)

Anyhow, your plan sounds reasonable -- bake for a bit longer than the original total recipe time to account for the decreased temperature, and then begin checking periodically for doneness.  (And as there are a number of pumpkin pie recipes that use graham cracker crusts, I assume they should be fine for most recipes.)