This shouldn't affect the bosses from boss trials, they are practically the same bosses that gets summoned as proven with the last boss event. They do drop uniques. About the drop rate, there is a fixed rate of which we don't know the exact percentage about. Getting an unique just depends if you get lucky or not.
You might have a higher chance with the boss event since they summon a bunch of them, then again it depends if you could get a boss or two for yourself by tagging them since first tag gets the drops (I believe that's still the case).
Wikis | ♥ S & A ~ 02-07-2009 ♥