As it is right now, Legendary status gives your dwarf a nice flashing symbol but not much else. A skill level 15 dwarf's work is only slightly better than a skill level 14 dwarf's, with more chances to produce a masterwork quality item, but still random.
I'd like Legendary to be more special--to give you extra options for dwarves that are Legendary.
So how about a "Masterwork" option on workshops limited to Legendary dwarves only? This would allow Legendary dwarves to work somewhat more slowly, use more materials (double?) but turn out Masterwork-quality products 100% of the time. It would be useful because sometimes you just want Masterwork quality only, like for your king's rooms or for your Forgotten Beast axedwarf squad--anything else gets scrapped. Using more materials and more time would be equivalent to having the Legendary dwarf just churn out items until you get a Masterwork, but without the need to get rid of the "failed" objects.
It makes sense to me for realism's sake, too, because a very accomplished worker can easily decide to produce only the very best--he slows down, takes his time, and scraps things before they're finished if he judges they're not good enough. If you order a Legendary dwarf to produce only the absolute best he is capable of, then he should be able to do that.
Optionally, it might not take more materials when working with metal, because the scrap from metalwork can be melted down into bars. For metals, it should just take more time and fuel.