EDIT: When mobs killed by potion, drop no loot (tested with Wither, dropped no star) Drops XP as per normal, dragon summons portal as per normal. No item drops.
EDIT: When attempting to test this in a previous version (1.6.2 release), the game crashed upon loading world, even after deleting command blocks. Crash source appears to be potions in inventory.
EDIT: When mobs killed by potion, drop no loot (tested with Wither, dropped no star) Drops XP as per normal, dragon summons portal as per normal. No item drops.