adam, as far as I know (indeed it is really hard to understand thoroughly):
- the ability to make a difference depends on: SP level (most crucial), your voting power when you vote, the level of payout of the voted post, and, I guess, the price of steem - this one is from my experience but not 100% sure.
- when I had around 2000 SP, I started to see a 0.01 impact (for a near zero payout post) but now my influence is almost the same - so I guess Steem price matters
- I actually want to propose to Steemit showing payout to third decimal place - letting the level to 'make a difference' 10 times smaller without changing anything. But I think this will affect minnows' perception and less frustration, which is really important. If you agree with me, you are welcome to write a post about it as I do not have time for several weeks to come. Actually steemstats.com already shows the impact of upvote to the thrid decimal place.
RE: At what point (in steem power) does an up vote have a financial effect