There is no limit to the DC one can add to a craft check to speed it up. Ever heard of the epic level handbook? High level magic items aren't supposed to be easily gotten at less then their sell value, which is why Craft is so time consuming. Realistically a person only spends 12 hours a day crafting, if you have a character that doesn't sleep, they can double the speed by doubling the time per day they craft, or could adventure with the group during the day and whip out a small personal forge from their bag of holding at night.