Current inventory W-78 warheads are the ones on US ICBMs, for example, and have a projected yield of between 335 and 350 KT, quite a bit less than the Castle Nectar device. Most of the really high yield tests were run with devices which were then (and still) too large to effectively mount on an ICBM, although the W-78's can be set up on a three warhead bus on a Minuteman III and still be delivered.
Treaty considerations limit what is doable more than technology, and with a three warhead bus, that's still about 1MT of yield on the missile.
Assuming (and that's a lot, but worst case and all that) that the DPRK could deliver a device with a 1.5 MT yield to the islands with an ICBM, and that if functioned efficiently (full yield, no fizzle), it could be one heck of a mess on that island. If it detonated at or above ground level which ever island was in the center of that would be pretty well wiped, depending on terrain shadowing effects. There would be survivors on outer islands, though (Hawaii is a chain of islands, not just one, but hundreds of smaller islands and the eight larger ones, seven of which are inhabited full time) and likely on the island that got hit as well. It would be nasty, though, for anyone there.
