I think shaft diameter has more to do with strength and weight than comfort. If you want a wood paddle with a thicker shaft you'll want to use a light softwood to keep the weight down.
I always thought I preferred a narrow shaft, but if the weight is right I like a wide shaft too. I also thought I liked a small pear shaped grip, but have found a bigger grip to be more comfortable, as it spreads the pressure over a wider surface area of my hand.
The protective lashing is a good way to increase the diameter of your shaft. If you go this route I would start the lashing a few inches above the blade. I just removed the lashing from a paddle and the wood had turned black (rot starting) where the lashing gets wet at the throat of the paddle. A few inches higher and the string will mostly stay dry when you submerge your whole blade.
