Robert as you say the efficiency does require about 20 times more heat transfered to the depths than energy created. This though I think is to OTEC's benefit considering it is surface heat that is driving storms and creating thermal expansion. At 1000 meters the coefficient of expansion of ocean water at 4C is half that of the surface.
The deep ocean also has tremendous capacity to absorb this heat. The study World ocean heat content and thermosteric sea level change (0 – 2000 m), 1955 – 2010 points out the total increase in heat content of the ocean over that period has only 0.09C. But if that heat were instantly transferred to the lower 10 km of the global atmosphere that volume would warm by approximately 36C.
As to methane hydrates they are found mostly in shallow waters on the continental shelf and thus are more likely to be impacted by surface heat than the movement of this heat away from where they occur.
As to the thermohaline, according to TAMU the Gulf Stream carries 40 Sv of 18°C water northward. Of this, 14 Sv return southward in the deep western boundary current at a temperature of 2°C. The flow carried by the conveyor belt must therefore lose 0.9 petawatts (1 petawatt = 1015 watt) in the north Atlantic north of 24°N.
To produce 15TW - about what we get from fossil fuels - you would move about 300 TW to the depths and most of this would occur in the Pacific, where the best conditions for OTEC and cyclones occur. I doubt therefore that the impact on this circulation would be significant.
Any problems however would become apparent long before we ever built out 15TWs worth of capacity.
As to cost, the heat pipe design reduces the size of the piping involved - the main driver for cost - from 15 meters for a 50MW unit down to 2 meters.
Thanks for your interest.