Using edshipyard I can answer my own question.
5D shields cost 190,000 and use 2.08 MW
5A cost 5,100,000 and use 3.64 MW
Thats for 22.5% extra shielding.
Good aguments for D modules, but it apply I think for anything wich aint is Shield Generator. For me, any ship, if wanna have best use of its shields, its supposed to have maxed shield.
If ship needed 1.56 more power, for that 22.5% shield to have much as possible, then its always worth to take and pay more for it.... Biggest shield that ship can equip, makes shields boosters more useful? Even from lower ones than A...