What tires for the 2015 Impala ltz with 20's

The factory 20 inch tires were Bridgestones. Fairly smooth when new, but not so much with more than 20K miles on them. I changed to Pirelli Cinturato at 33K miles. They have close to 50K on them now, and will probably need changing before next winter. I don't know what kind they will buy next, because it belongs to my daughter in law now. Pirelli doesn't make the same tire now, but has a new touring tire. That's probably what I'll recommend now, but they may want to spend less. That particular size is made in their Mexico factory, and the quality control was not good when I bought them. Seven tires to get 4 good ones, if you can believe that.

I'm looking at the General Altimax RT45. It's apparently not easy finding smooth riding tires for this car. I'm guessing the large size doesn't help. I've read some people have installed the smaller wheel and have a way smoother ride now.

Yep, the shorter the sidewall section the less deflection a tire will have when you hit a pothole or speedbump.