Thanks for your collective input...my father was also an Impala owner for two models, but has now switched to a Malibu Maxx. He was pleased with the 3.4, which he described as more the adequate for most situations and was quite surprised at the fuel efficiency. He also has an Aerio, and was shocked that the Impala gets better mileage!
Anyway, I arranged a test drive for tomorrow, so will let you know what transpires. If it drives well, then I'll treat Mom to a CarProof report and, pending results, see about making an offer.