I am looking for for *statistical evidence* that an easier to read
(simpler copy) web site leads to more sales. A university study would
be nice, but anything that is reasonably persuasive and uses test
subjects. I need to make the case that conversational copy (like blogs
have) is better for winning over customers than corporate-speak (like
a typical press release).

I am not at all interested in general unproven assertions (even by
noted experts), I have found hundreds of those citations. "everyone
knows simple sells!". Ugh. :)

A fallback position might be a controlled study where response rates
for brochures or email were greater when more conversational/simpler
copy was used.

Another fallback could be a correlation between higher click through
rates and simpler copy.

FYI: Here are two (of several) measurements used to determine how readable copy is:

Gunning-Fog index
Flesch Reading Ease