The OEM Michelins are very good on fuel economy. You should notice a slight improvement with your new tires. But, at this point, you may not know if it's because of the new tires or your clean air filter!
I don't know much about the Regal, but I can imagine the suspension in the Accord is stiffer/sportier than the Regal. That would explain why you're feeling more of the road. How does each car feel in a fast corner? The Accord more sure footed and the Regal tends to roll to the outside and float a bit?