At best I would think it could be a "YF" but even that seems like a stretch to me... I am somewhat disappointed when I get a Maxima as a premium rental after experiencing the Lacrosse, which is superior in every way. Impala is between a basic and midrange full size car depending on trim level, but certainly only the LTZ or SS trim could even begin to pass for an entry-level premium sedan.
Personally I wouldn't ever accept an Impala for a premium rental unless the only alternative was a grand marquis!