All dampers (all makes) use oil which is forced through valves to create the damping action. This action causes the oil to heat up and if the oil gets too hot, it can boil and the damping effect deteriorates.
To prevent this, there are a number of things you can do...
1. Use a damper design that carries "too much" oil.
2. Design the valving so that the oil is always traveling in one direction, irrespective of the wheel's direction of movement (up or down). This ensures the valves always work with cool fresh oil, not oil that has just been heated by the valving.
3. Pressurise the oil to raise it's boiling point.
When people describe dampers as been "gas" units, it means they use option 3. Most do this by pressurising the whole unit and then sealing it (as the OE units do), but if you mount the gas externally and have the cannister feed just in front of the "bump" valving, you can get a secondary effect. As any gas is slightly compressible, mounting the cannister here can let the gas slightly cushon the initial shock of the wheel movement, allowing a more linear acceleration of the damping oil.
If you think about the work the damper must do on a gravel rally or rallycross car, there is no comparison with the work done on tarmac.
In the real world, driving on the road, it does make the system slightly more compliant and gives a small increase in grip on bad, wet roads, when compared to a non gas unit. Option C has a slight advantage here.
On a track, wet or dry, the suspension is distributing car weight and controlling body roll, rather than soaking up bad surfaces. The control given by option B units makes these units the preffered option.
Wet grip is as much about tyres as suspension but in the wet you want soft compliant suspension and a small amount of body roll. I always back my dampers off if going for a blast in the rain.
Only Leda and Proflex units use all three of the above ways of maintaining damper performance.
I discussed suspension dynamics in the last True Grip. If you aren't in the SIDC, join and ask for a back issue of TG3.