Here is some previous years threads with data:
My 2009 prediction & results of 50% Alliance #1
2008 with some '07 & '06 data
2006
The Philadelphia Regional is an interesting case as far as #1 seed vs the field. Last year
I successfully predicted that the #1 seed would win Philly due to the new seeding algorithm (also contains my 09 spreadsheet). This a major streak breaker because before 2010, no #1 seed had won Philly since 2001. Philly tends to have a large similar top tier of offensive robots and lots of defensive bots that causes a lot of upsets during quals and elims.
I don't have the % of #1 Alliance champions in 2010, but I suspect it was higher than usual. The scheduling algorithm (as much as I disliked it) did a good job of seeding the top robots high regardless of schedule difficulty. This year with a return to WLT system, I have been more interested in the how OPR correlates to event winner. The team with the highest OPR during the quals almost always wins the event even if they are not in the #1 alliance (won all the Week 1 events). In Chesapeake where the top team didn't win (one of the few cases), the alliance with the highest combined OPR did win.