Looking in-game at the description on DQ IX for "Mortamor's Map Lv. 1", I see the "Treasures Plundered" values as having Midi medals dropped at 100%, Somnia gloves dropped at 20%, and Yellow orbs dropped at 10%, not 5%.
As I have not leveled this legacy boss up yet, I cannot confirm the increased drop rates for higher levels. However, a Hoimi Table set with 'Red: 10%' (for the orb) does seem to confirm this 10% rate.
Is someone able to confirm?