plain old rolled oats are very different from steel cut oats. Steel cut oats are almost like pellets, very hard and take about 30minutes to fully cook. Rolled (not instant) take about 5 minutes to cook.
Honestly I am not sure about the digestibility of steel cut oats that aren't cooked. easier digestion and absorption of nutrients is all based on how much surface area you can create from the food you take in.
Look at it like this.
If you eat whole kernels of corn, didn't chew them, just swallowed them. When you go to the bathroom the next day they would be completely intact whole corn kernels, so you basically got nothing out of them because you didn't digest them. I would have to imagine steel cut oats (raw) would be the same way.
Rolled oats are basically steel cut oats only rolled flat which makes for a greater surface and easier digestibility. on top of that you are chewing the raw oats which in turn creates even more surface area which makes for easier digestion and absorbability
Raw rolled oats I believe is definitely the way to go if you are going to put something in your shake. Unfortunately I would have to believe that raw steel cut oats are passing right through you without you absorbing much if any nutritional value