Stan: Yeah that sound about right the minos 2 puts out 14kw of heat and the wetback from memory does not sit in the fireplace rather attaches to the back of the fire box
To get a tad more heat performance out of it putting it at the back inside the fire box and copy the design of the metro LTD wetbacks LINK (at your own risk).
I would suggest a low pressure open loop system rather than having a heat exchanger its just too inefficient from my experience.
andrewNZ: Seems to me you'd be adding more things to go wrong, with little to no benefit. If you're using an old cylinder, perhaps you can find one with a wetback connection already included.