I dont know if grooves on the drum would vent gasses away & improve the grip of the shoes on the drum, but why would we need to? Stock, mine would lock up with just 3 wet turkey feathers laying on the brake pedal. YES~! I tested that,,
What? You dont believe me? I am crushed.. just crushed.

Anyway, it would lock up with such light pressure I couldnt use it "Hurriedly" w/o sliding the tire, so I modded the brake pads & now I can push on the pedal a good bit more w/o a lockup.

You have not made clear the object of the exercise. Are you trying to decrease the required pedal effort? Are you trying to increase the required effort prior to locking the wheel? Or do you have something else in mind?

In any case, whether disc or drum, the rear brake doesn't contribute a lot to the total stopping ability. Given that the front brake is used, the rear may possibly contribute up to about 1/3 of the total effort, on a very-long-wheelbase bike such as a raked cruiser. On a short-wheelbase bike such as a sportbike, the rear brake is almost useless if the front brake is properly used.

I took the edge of a grinder disk to the shoes & made 4 or 5 grooved across the shoes, decreasing the contact area by somewhere around 1/3 or a bit more. I tapered the leading edge of each area a bit. I like the rear brake a lot better now. It will lock up, but I can feel the pressure on my ankle, whereas before, I couldnt even feel the pedal press against the sole of my boot. I like to keep the pedal high, very little play before the shoes hit the drum.

This way, if a mod isnt good, the hub isnt screwed. just the shoes.

Be sure to use appropriate safety measures. Grinding on brake shoes could be less than healthy, ya know??? Eyes & Lungs,, IDK if they still have bestasses in them or not, or is that asbestos?Whatever, dust from brakes isnt likely to be a good thing to breathe. Dont cheap out here, okay?