Are there any significant values to (non-Castle) Boss Kills?
Sure, they drop a lot more than a typical mob. But they're also usually a lot harder/slower to kill (unless you're abusing an Infinite/Automatic Scroll potion, anyway). And given the number of regular mobs you could have killed in the same time instead, I'm not convinced this is a win, even then, either XP-wise or drop-wise.
Further, I've not even noticed any Achievements (nor points) related to Boss Kills.
So, putting the above together, it seems like the "Random Boss Fights 20% Chance/Room" potion is mostly a "slow gains down" potion.
(Or alternatively, a "why are you slowing yourself? Stop slowing yourself!" potion.)