I suspect the distribution of SP on Steemit will be comparable to Karma on Reddit. In 2015, the top 1% of Reddit users had 47% of the total Karma.
If you can't see the following photo in detail, click this photo link: http://imgur.com/cfWa4Ty
In an older data set, the top 1% of users had about 20% of the Comment Karma (different from Total Karma):
Logarithmic Scale:
In this logarithmic graph, each number on the y axis is 10x the previous number:
1 = 10
2 = 100
3 = 1000
4 = 10000
5 = 100000
6 = 1000000
The problem with Steem's voting reward algorithm is not that 20% get 80% of the rewards, but it is that the selection of the 20% is done by 1%. And this is motivating the wrong behaviors and focus of content produced.