Rarity / Quantity balance - Is there any soft cap?

Hello,
I'm running 400% rarity and 60% quantity and I'm gonna add/switch some quantity stats to that.
So my question is, from which point rarity start to be less efficient.

I would like to balance my mf stats and i m pretty sure that droping to 350% rarity to get 100% quantity is a good thing, but if i hear that rarity start to be useless after 200% i ll add some more quantity, or balance with better survivability to run map higher than lv72 (haven t tried higher level map yet).

I might be not clear so let s make it simple :
What is a good MF balance? choose an option :

A: 400% rarity + 60% quantity (actual mf stats)*

B: 350% rarity + 100% quantity

C: 250% rarity + 150% quantity

C. More mf rolls = better chances.

http://www.pathofexile.com/forum/view-thread/725812
IIQ/IIR: Balance and Diminishing Returns miniguide (v1.0.1)

good read

Who invented IIQ by the way? GGG?

Thanks a lot Thorien_Kell, i made several research on forum and never found this post, it answers exactly the questions i was asking.