I don't think anyone ever refers to "Peel" as an area. Proper usage would be "Peel Region" or "Region of Peel" or "Mississsauga and Brampton" (seeing as Mississauga seems to be eyeing secession again).
But in southern York Region ppl do tend to think of the three southern municipalities more akin to being a single city more than Mississauga and Brampton is. Probably because of the way the cities there have streets as borders with no sense of separation.
I wonder if Peel would be dissolved if Mississauga separates, as it wouldn't be very viable with only two municipalities.