Personally, Ive played with them a bit. Bit Ive never found them that practical. They really boost the efficiency - and hence heat output - of a Fuel Cell, but also restrict the directions you can distribute heat out the cell.
As such, they are easiest to work with by designing a reactor where the heat gets pumped into and out of the hull, rather than distributed from the fuel cell into surrounding components.
i.e. if we surround a Quad Fuel Cell with 4 reflectors - that will be the base efficiency of 3, +4 combined for each of the reflectors for the maximum attainable efficiency of 7x.
This is going to be dumping a massive 448 heat into the hull however.
The cooling component with the biggest hull draw is the Reactor Heat Exchanger: it pulls 72 hU/s. But you need to get the heat out of the thing which is complicated. The next best is the Overclocked Heat Vent - which draws 36 hU/s from the hull, but only vents 20 hU/s. We can fix this by surrounding each one with Component Heat Vents - which can draw 4hU/s from each surrounding component and vent it. 4 Component Heat Vents would together vent 16 hU/s - exactly the excess we have in the Overclocked Heat Vent.
If we tile that into a grid, with the Fuel Cell and Reflectors in a corner, we can clear out 432 hU/s from the hull - leaving another 16hU/s to cater for: so we can just cram some Reactor Heat Vents into the gaps along the edge to cater for that.
Here it is in the old reactor planner. A modern IC2 version will have the Uranium Fuel cell last twice this, so 56,000,000EU is your total output from a single quad cell.
Code: 21p7gh3nrs9mn7okyiq3cp5hhy6znr813z6hve7xkcc0n13tk7akwft0d3r100l9407bgd8iz88k268 (Also works in the new planner)