The 180 that I flew had the Robertson stol mods on it, and a seaplane propellerwhich I believe is a bit longer than the normal one. (my wife could always tell the sound of this aircraft when I flew over our house.) I spent lots of time in it over about a two year period, doing low level survey, and generally commuting. I often crossed the country in it. I expected to get 144 kts TAS on cruise, and generally did.
Bushy
Did your 180 have spats ? I believe they added 5 knots or so.
There was one 180 in Whyalla VH-RFF that was a good 10 knots faster than it's two sister ships RFD and RFE. No one could explain why.