There's dozens of those things all at once and it'd be ridiculous.
This mod should be fully compatible with UCP 4.
It probably wouldn't be difficult to nerf the drop rates in general during Slaughterdome missions or rather, in the Slaughterdome areas.
Replay Borderlands 2 with this big overhaul mod
This version enables spawning of basically everything from the beginning, including all grenade types, relics, class mods, rocket launchers, etc.
Description: Boss Unique Drop Quantities - This section lets you define how many unique drops a boss will drop.