I've been in a serious time crunch while travelling since the GT6 event, and didn't have time to collect these statistics for you. Sorry.
The "users online" metric reported in the forums is not an accurate way to judge traffic to the site.
When GT5 was formally announced at E3 2010, GTPlanet received 64,384 visits in one day. On May 15, 2013, GTPlanet received 166,825 visits, although this number would have been much higher had the site not been knocked offline.
The 4x vs. 20x number I referred to earlier was in reference to the number of simultaneous page requests, the real metric which I have to worry about in terms of the server infrastructure. Because I was the only one doing the live blog, when I announced that, it spread incredibly fast via social media. Page loads per second increased exponentially until the application server collapsed.