The only loot I've ever gotten from defeating bosses are consumables, like the type of stuff you can buy at the start of every day using your Badge Points.
If I return to the game after some time away, the screen is packed with notifications about the loot that the bosses have dropped in my absence.
Strategy-wise, I don't think 1% more boss loot is really worth it - there are a couple of these items that can be useful (ie, Cripple Spawner for instance) but most are minor, short-lived bonuses at best.