Hey Mark,
As far as when a cooking banana is ready from a chef point of view, if the cooking banana is not soft to the touch where you can press your finger and slightly squeeze it, its not ready no matter what color it is....
A trick for getting rid of the starchy taste, You can marinade the cooking banana in a honey and lime mixture..
Honey mixture:
Add 1 tablespoon of a good honey I use tupelo honey to a half of small to medium lime, squeeze it and whisk it. The marinade should have a loose consistency and taste for preference. It should be slightly sweet. you should be able to coat 3-4 7" size Banana half's.
Prep
Cut the cooking banana in half and make a small slit down the middle where the lil seeds are then brush the honey-Lime mixture a few times getting the mixture into those slits is the key.
Then place it in a bowl for a few hours and cook on medium-high heat. Do not mash them just turn when there golden brown 3-4 minutes depending on your stove, as each brand varies in heat variance. Let them sit for a minute and brush them again for added flavor that gets cooked off and they should be good.
Let me know how it goes....
Big Tony