Now I've read that affection with companions affects their crit/efficiency rating, but does anyone know how much affection you need for this to be decently noticeable? I've been searching the forums a bit, but I couldn't find a decent answer to this question.

Say, would it be noticeable with about 8000 affection?

simple for every 2k affection you get +1% crit chance. that means at 10k (max) you will see a 5% bonus to crit chance. Considering that the base chance is either 15% or 20% (only bioware knows for sure), that affection bonus is either a 33% or 25% increase in the chance to crit. That's pretty good.

Throw in an individual companion's crit chance bonus and the legacy perk and you are approaching or breaching the 30% chance mark.