It's still a 40A circuit on a 40A breaker, it just has wire rated for 55A along most of the run. Using a couple feet of 8ga at the end will not hurt anything.

You already downsize the massive 200A cables coming into your service entrance with 15-20A wires going to most of your devices, then those devices may have some tiny 20+ga wires inside them. You use what you need for the individual load, you don't just carry on the larger cable for consistency.