I just ran the stats for the tournaments since the 1994 expansion to a 64-team field (but not including this year's games), and this was what I came up with (winning percentage of higher-seeded team):
- 1 vs. 16: 99%
- 2 vs. 15: 100%
- 3 vs. 14: 100%
- 4 vs. 13: 94%
- 5 vs. 12: 78%
- 6 vs. 11: 71%
- 7 vs. 10: 66%
- 8 vs. 9: 48%
For the top 4 seeds, an additional factor that has surely boosted their winning percentage is the home-court advantage they've enjoyed in the majority of those gams.
For seeds 5-8, the vast majority of their first-round games have been played on neutral courts.