Like others said, that was likely all settled in the pan, If that much debris was in repeated circulation past the filter, the engine would be gasping and wheezing, audibly and visibly. At the very worst, the oil pump got some scoring, but a single pass of those particles, being so large that they'd have a hard time getting wedged in the tightest spots, may be just fine.
The dealership, unless they see some sort of evidence of damage (gasping, wheezing, low pressure light) won't have cause to do anything from the reports of the flakes themselves.
The MPG is a separate issue, and perhaps harder to track down, based on board histories. Someone's gotta read some real-time ECU data to look at that. I've also seen cases online where new cars take a while to get their mpg up. Check tire pressures, air filter, and tire wear?