I have always beleived that Suppression really does give you +10% hit on all of your affliction spells. However, I read somewhere that due to some wierd mechanic, the result of this talent is only effectively 3% or so hit. Is that true?

Next time you have a one specific question, try to ask in a class talent thread, such as:
Warlock dmg at lvl 60/70: afflic/demo/destro - Theory.
However, if you had a more general question, then a thread for that seems fine.

When I was Afflic, suppression gave my afflic spells 2% more spell hit per talent point spent.