Sure, why not? Brass is just easier to work with. From a practical point of view, when I'm filing and sanding the excess you want the filler erosion to be faster than the steel tube. I don't want to cut into the steel any more than I have to. Harder to tell when you're filing steel on steel.

As you're alluding to, strength comes from the properties of the material, as well as mechanics of solids (shape and size). You can make something stronger by using stronger material, or more of a weaker one. My patch is > thickness than the base material (tubing wall thickness), so, yea, I've got a bigger cross-section as well, per plan.

Someone put a hole in that tube. Not sure anyone would agree that this doesn't make the frame weaker. You've seen many frames drilled out in the same manner without issue. I've seen 2 where it eventually cracked the frame. Here's a third (maybe you missed this one).

I often put holes in the bottom of the down tube to run a wire from a front hub generator to a tail light. I always silver braze on some material first (often a very small washer) to reinforce the area around the hole. It's probably stronger than before.

The OP's frame has a hole drilled into it without any reinforcement. It's got to be weaker at that point, can be a stress riser location, and a place for moisture to intrude into the frame, especially at a place where sweat is dripping down from the rider (I'm sure you've seen many frames with rusted out top tube cable guides prove out that point). Maybe the frame will be just fine, maybe not. It's just bad practice to drill holes in frame tubing without reinforcement.

Since the OP's repainting himself (I'd bet for less than $1000), I'd get it plugged first.