Comparing 2 horses from different errors is subjective but to Lance’s comment about Sunline was race specific (Auckland Cup).
I have read, listen and noted that the 2 miles at Ellerslie is less taxing than Trentham or Riccarton. Front runners like Jan’s Beau, Kotare Chief and that Aussie horse Cooksley rode and decided to go to front at the 1200, could dominate. I think his point was Ellerslie was a course Sunline could have got away with a 2-mile win.
As for Show Gate. Not sure if she won at group one in Aussie and I can only think of 3 times she may have won at group one in the north island, though she did race against a stellar group of race horses and beat them.