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I've seen a similar improvement in all kinds of stats (sometimes including ranking) by using a caching plug-in for Wordpress.
Page load time is mostly NOT determined by googlebot - the core data comes from real visits by browsers using the Google toolbar.
I don't think it matters that much for rankings. It's really a go/no go situation. Just because your site is faster than everyone else doesn't mean you will be #1.
However, a faster page and site is better for users and helps conversion.