The mini goblin melter works! Praise Armok!
X^X
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Key? (X and # indicate what? And is the pressure plate hooked to the inlet of the pump or the mechanism?)
I'd worry about building destroyers. The limiting factor here would probably be how long the magma takes to drain off of the pressure plate. I wonder if this would be better (looking at it from the side alternating floor/ceiling and main tile):
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█ ████
█~████You need to build the bottom pump before the top one. Magma goes in at the bottom, and the magma return is at the top. Both pumps are always on (or rather, the machine runs only when the power to the bottom one is on.) The pressure plate is hooked to the hatch, and triggers on hostiles only. When the pressure plate triggers, the hatch opens, allowing the bottom pump to draw magma onto the pressure plate, cooking the goblin. The top pump then pumps it out again through the grate / floor bars. (Again, the bottom pump must be built before the top one - pumps pump in reverse build order, so what'll happen is the top pump pumps out the magma from the pressure plate, and then the bottom pump immediately pumps the magma back in again.)
Cycle time with this design is 0 ticks / 100 ticks. (Potentially 1 / 101, depending on build order...)
Untested, though. And make absolutely sure that the bottom pumps cannot have power unless the top ones are already running, lest you have a bit more !!fun!! than you anticipated.