I always though the rationale for not using ballast in freezing
temperatures was the danger of the dump valves freezing, not the
ballast itself. It would be a rude surprise to enter the pattern at
Ely to find out that one wing valve iced closed and the other didn't.
(Or worse, landing out in a tight field without that long Ely runway.)

On the original topic -- I can see how the sink rate has increased at
altitude, but the shape of the polar stays the same, right?

..02NO