Finding Royal Circlet...

[the Langolier]

So people who do just Laz runs tend to shortchange themselves with the items they can get.

Not exactly. Normal monsters only have a 10.7% chance to drop an item, a minimum of a 36% chance for that item to be magical, and a 2% chance for that item to become unique. Unique monsters, OTOH, will always drop an item which will be magical, and that item will have a 16% chance to be unique. They also have fewer base items they can drop.

For normal monsters to drop the RC (who are also able to) there is a 1/114 chance for the item to be a crown, rendering 1/114 * .107 * .36 * .02, or ~0.000675% chance to drop the RC.
For unique monsters we have: 1/76 * 1.00 * .16 = 0.211% chance to drop the RC.

Assuming 185 monsters on a level and every one of those is able to drop the RC, we get a 0.125% chance to drop the RC. This can't compare to just one unique monster, and on level 15 there are always at least 4 of them.

Of course, killing all the monsters on a level significantly improves your chance to find any item you may be searching for, but Laz runs are definitely the fastest way to find the Royal Circlet.