There's one other source for this data - Noam Wasserman at HBS did an extensive study of startups. His data (in The Founder's Dilemmas) shows that companies that make a "quick and easy" decision to split 50/50 received significantly lower first round valuations than teams that spent more time debating the split (although it then made no difference if the long-negotiated split was equal or unequal). He also believes anecdotally (but doesn't have data) that the quick-equal teams are less successful long term.

Great book, incidentally - this is from page 163, I highly recommend the whole thing.