Both speed usage and energy consumption are exponentional.
Speed formula is "Machine operation time" * 0.7^N and consumption is "Machine operation energy cost" * 1.6^N [N = number of overclockers]
so both increases exponentionally, but consumption increase is higher if compared to speed gain.
Macerator/extractor/compressor needs 16 OC for max speed (1 operation per tick) and will consume about 5500 EU per operation
Elec furnace needs 13 OC for max speed and will consume about 1300 EU per operation
Recycler needs 10 OC for max speed and will consume about 128 EU per operation
so to put it simple terms, as long as you can afford the power, you get your worth out of increased speed?