Not so sure about the behind the scenes "automatic router". What's to stop OpenAI from slowing gimping GPT-5 over time or during times of high demand? It seems ripe for delivering inconsistent results while not changing the price.
That said, I've had luck with similar routing systems (developed before all of this -- maybe wasted effort now) to optimize requests between reasoning and regular LLMs based on input qualities. It works quiet well for open-domain inputs.