I have noticed a difference with as little as .002. Not huge, but could feel it. And .004 loose was worth 10 PSI cranking compression and an inch of vacuum on one combination. You're not supposed to go much looser than the spec, I've heard .002 is OK for most, some might tolerate more.

My understanding is tighter is not an issue as long as its closing and there is some lash. Although every .001 tighter you go is going to add more duration than the last .001. The slower the lobe the more the lash is going to effect it (more duration for every .001)

Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street