I've done a bunch of experimenting since I posted the original post. Here's what I found:
The firesteels throw sparks roughly in proportion to their diameter – after the coating is worn off for about 1/8 of the diameter. I think the micro-flats that form are responsible, and larger diameters just have more micro flats in contact with the striker. This makes a BIG difference with natural tinder that is "freshly harvested". If I put in the garage to play with later, it only makes a little difference.
Longer firesteels are a little bit easier to generate lots of sparks. I think it's because you can see the results and correct the angle so that you are making maximum sparks by the end of the stroke. With enough practice, maximum sparks happen even with short strokes / rods.
The LMF, BSA, Doans & Coglans bars all seem to be made of VERY similar Ferrocium. ie: You can scrape with medium pressure at medium speed and get sparks. The sparks are not super hot though. The "mischmetal" rods being sold that advertise they have a significant magnesium content make much bigger & hotter glob-like sparks. But you have to scrape both hard & fast to get sparks at all.
So for young scouts, I find they can't spark the Mischmetal rods reliably. For starting fires with natural tinder, the LMF scout is the minimum the younger scouts can make generate enough volume of sparks to eventually start natural tinder. The LMF Army size is easier, and lasts a LOT longer for scouts (who make fires more than once a day). For cotton balls, the BSA hotspark works just fine and fits in a pill bottle that is just long enough to hold some wooden "kitchen size" matches.