Alright, I decided to swallow my fear and give this a try; forgive me for being something of a forum noob, but I read through the entire thread, and decided to attempt crafting a solution that could (hopefully) satisfy the needs of more efficient CPU usage; more accurate attempts at simulating mining/quarrying and the resulting products; and the current problems with legendary versus dabbling miners. My suggestions, however, require a complete overhaul of how mining works and how certain professions interact with them. On the plus side, however, I aim to avoid the whole improved hauling conundrum altogether. Hopefully.
First, instead of having one Designation, there are now three available for mining: Pulverize, Mine, and Quarry.
Pulverize is a setting that digs out the designated square without creating any stone. However, if one was to create a "Gravel Pit" (a new workshop; I know adding another workshop is less than ideal,) then when a dwarf smooths a Rough Stone Floor, the game handles the aggregate in a similar way to the "meta-bars" that are created when a player melts an object but lacks enough metal to make a bar. In other words, all aggregate is treated as a single integer stored within the Gravel Pit workshop rather than a series of objects that have to be hauled and tracked. This solution then doesn't create any new game objects that have to be tracked except an integer without a map location or anything. It doesn't spawn troublesome hauling jobs; it simply increases the integer stored in the Gravel Pit.
Now, I understand that creating a new object without an implemented use is an overall bad idea, so then, aggregate will then be used for creating roads and constructions. When a dwarf goes to construct a wall, or a road, they simply subtract whatever arbitrary integer Toady or someone more anal than I decides on per square. Ideally, I would like that simple gravel roads wouldn't require an architect, but that's a whole other discussion entirely and beyond the scope of this post.
Mine is more or less exactly how it works now: Inexperienced miners have a chance of leaving behind a Rough Stone Floor, instead of a stone, while legendary miners always produce stone.
Quarry is the final designation. When a miner digs out a square designated as a quarry, it attempts to make a rock block on the spot. Inexperienced miners have a chance of digging out a stone instead of a block, but legendary miners always produce blocks. Quarrying will never produce blocks out of gemstone (though it can create blocks of ore...) but instead will allow inexperienced miners the ability to produce gemstone and ore despite their relative inexperience.
As one final comment before I disappear whence I came, the different designations require different amounts of time to perform: pulverize is the fastest, mine the current speed, and quarry slower, all being used to represent the amount of care and accuracy the miner is attempting to produce the various products.