You are generally better off with shield boosters if your ship has more powerful shields, because its a percentage bonus.
Shield cell banks add a specific amount of "shielding", so will add less "shield rings" on powerful shields.
AFAIK shield boosters dont add to other shield boosters, that is 2 A boosters will add 40% (20% each) not 48% (20% + 20% + 20% of 20%).
Or now I've wrote that above and I don't really understand it:
shieldStrength*(1.2+1.2) not shieldStrength*1.2*1.2
They each add a percentage of your base shield, to your base shield, to produce your final boosted shield.
Ex : if your using A5 shield generator on a Vulture, you'll get a shield of 344 MJ.
With 1* A0 shield booster (0,20), you'd get : 344 + (0,20 * (344)) = 412 MJ
With 2* A0 shield boosters (2*0,20, or 0,20 + 0,20), you'd get : 344 + (0,40 *(344)) = 481,6MJ
etc
So basically, shield boosters effect is additive between themselves (hence the 0,2 + 0,2) but multiplicative towards the base shield (hence the 344 value). They do not apply their effect to a "
currently boosted by one power booster shield" - one such thing does not exist.
In other words, the
total boost value is calculated first (here : 0,20 + 0,20), then it is applied to the
base shield (here : 344 MJ). It gives you the value of the boost (here : 0,4 * 344 MJ = 137,6 MJ). That you add to your base shield, to get your final boosted shield value (here : 344 MJ + 137,6 MJ = 481,6 MJ).
Also, what Heavygroovez said.