I think a lot of the arguments become moot if quality-vs-time becomes something defined from the manager. Then it's not the craftsdwarf deciding to churn out cheap stuff, it's the manager saying "Do this fast or else." or, "The King needs the best stuff, take your time!". I'm still not sure if there's a point as far as speed goes, a highly skilled dwarf will spend maybe a couple ticks in the workshop and twenty or so hauling the goods in from a nearby stockpile, so there's no expediency benefit, so I suppose the only point is to reduce quality.
On the other hand it might be simpler if it just wasn't necessary from a gameplay / theme perspective. Thematically we can assume that every dwarf is always trying to create a masterwork item, but there's just no reasonable way to skip a step. A masterwork item isn't a result of extra work, it's pure luck of how a chisel chips away at stone; the only extra work that improves say, a stone mug is lodging gems and precious metals into it. So, telling a legendary dwarf to work faster isn't really possible, the only effective thing would be telling them to do deliberately shoddy work; no crafter worth their salt would do that, dwarf or no.
From a gameplay perspective, we could start by making craftsdwarves not care so much about their own masterwork items and works if they have a lot of them already, so we don't have to worry about defending every one of 200 masterwork items from being stolen by kobolds or having a very valuable dwarf go insane. This might make things a bit easier because it would lessen the difficulty of keeping legendary crafters happy.
Introducing a more freely sliding scale to the /value/ of such items, however, would shore up the difficulty. The hundredth masterwork slate mug will be worth perhaps 1/10th as much as the first, with correspondingly less value for lesser grade crafts. Value for each civilization would be determined by how many the caravans have already acquired, determined each time they bring the goods back to their native civilization. The value of the goods inside your fort will be based on how many are in your stockpiles, meaning if you have tons and tons of expensive mugs, they'll be priced down enough for your non-noble dwarves to afford them.
There is the question of whether it's worth the complexity. It could probably be abstracted to make little difference performance-wise but that may not be worth it to a lot of people, and others may want to shut it off because they don't want to deal with the idea of inflation in a videogame.
tl;dr version: instead of telling dwarves to stop making masterwork items, just introduce a sliding scale of value so if you have thousands of masterwork items they won't go crazy if one is lost, and workers will be able to afford them. I really didn't get into the idea of spending more time to make better quality stuff, that's a different issue entirely.
It occurs to me that there already is something of this in the system, the price of items will change on a percentage scale. It may be based more on raw quantity than average value per piece though.