Actually, in this case, 99% of the time it will be a trash mob, because instances are 99% full of Elite trash. Bosses represent less than 1% of Elite mobs in instances.
I am just speculating, but wouldn't a target level check be a more viable option ? Like if target > X Levels than the player & Elite then it's a boss, else it's simply a too high level reg mob. Actually, I remember RareEliteKiller plugin had a level check so you can see there how it's made and combine it with an Elite check and you got your universal Boss check.
Boss ID's would be actually only useful if whatever OP wants to develop will implement specific boss-specific things. Like a bot that automatically fights Ultraxion for you, stacking with the group, pressing the purple orb every time the Boss begins to cast Hour of Twilight.