I am a crusty old crumdgen that has watched this for years and the compromise I have come up with is having thumbs a little wider than shoulders.
After having a number of students come back from World Class paddling too close together. I started looking at images of good paddlers online young and old, slalom racers, flatwater racers, playboaters, and general paddlers. The old farts were all 90ish Dane is close to 90, Levinson closer to shoulder width, Ben Marr halfway between, Some slalom racers were less then shoulder width but most were closer to 90
the trade off is power vs dexterity. Wider more power and ergonomics. Closer speed and dexterity.
Wider grip gives more power. To use the lever analogy - your bottom hand is the fulcrum or pivot point. The closer it is to the blade the longer the lever is to your top hand. You can also think of it as the farther away you move your hand the longer lever the water has. Often mistaken that having to pull harder means it is more powerful. Wider hands is like granny gear, Narrow hands is like...you know the other gear where you have to push harder. Flatwater racers are generally closer 90 degrees.
Narrower allows for more dexterity and speed. Your hands don't have to move as far so they will move the paddle faster. Of course this allows for you not to use your body as much. For example you can get your paddle pretty close to a bow draw position with out turning your torso at all if your grip is really close. However you have no power. Your shoulders are bent too tightly and you are just using your forearms.
Which brings me to what we call in the business "...being stacked" One thing to take into consideration is joint alignment and the paddlers box. Not only does this protect your shoulders but keeping your wrists in front of your elbows and in front of your shoulders provides power and less torque on joints. having a wide grip forces your to use your body more. That takes time but you get the power back out. Pretend you are holding your paddle like normal. You probably have a natural forward bend at the shoulders 40 to 50 degreesish and a natural bend at the elbow 30 to 40 degreesish. Now pull your hands straight back to compare them with your shoulders and you will find you are a couple inches wider than your shoulders.
friend of the fork, knife, and spoon