It's not only the GI. There's also the II, or insulin index, which I'm sorry, I don't know much about, and then you have to take into account the NUMBER of carb grams taken in, AND what they're taken in with.
It's unlikely that ONE banana will cause much of an insulin spike at all, and while they do have glucose in them, they also have fructose, which, as we all know, is a Very Bad Thing even at the best of times (which, heh heh, is probably post w/o, actually), thanks to its preferential desire to go to LIVER not MUSCLE cells.
You'd be better off having a cup of oats in your shake, or 60 - 100g dextrose or something like that if you're not too worried about subsequent blood sugar drops and carb cravings.