I found a web page with some basic information on reservoir shocks:
http://www.atvsuspensiontech.com/reservoirs.htm
my interpretation of this in an rc context is:
considering that most rc's are traction limited the reduced mass of the basic shocks may improve the agility of the car, whereas the reservoir shocks could improve handling on bumpy terrain like cobblestones, pebbles, rocks
by virtue of it's increased volume of fluid and gas making the shock more plush without
sacrificing overall damping
they may be a significant upgrade specifically for the two stroke cars that constantly transfer vibration to the suspension system and counter the effects of that warming it up, creating wear and reducing overall traction
for the electrics:
honestly they seem only situationally useful and they increase the total mass and sprung mass of the car which would seem to make them a tuning option rather than a straight upgrade
from a purely practical point of view, money is probably better spent on tires, steering and power upgrades before worrying about total oil volume
however, they do look cool and may come in handy tackling extremely rough terrain
bare in mind, that's my opinion and not any kind of empirical fact
if anyone has them do tell, i'd like to know if there's a noticeable difference
over stock on any specific terrain types and i'll keep a note of it