Once I made an IPA predominantly with Amarillo, never again, tasted like Apricot nectar, which I am NOT a fan of.
Less is more I find, I have done a series of American wheats over the years, all single hop, and the Amarillo one was sublime.
Heat close to or at boiling will degrade hop flavour components. That's why you get more hop flavour from late additions than early ones - less contact time at high temps.
Whirlpool additions are essentially very late additions at a temperature lower than that of the boil, less of the volatiles...
When I sparge I always get 80%, when don't I get around 70%. Always the same results over about 50 bigw biabs.
Whether I do it or not depends on how much I need the extra gravity for a given beer and if I can be bothered, which isn't often these days!
I brewed a kolsh a while back and got 2 cubes and 4 litres out of it, so I put 6l of boiling water and the 4l in a 10l cube and fermented it as a starter for the normal cubes.
Usually I work it so that I'm left with 6-7l of beer and then i get a 3.5% beer while growing the yeast. Those beers...