I need to point my customer to IIS 7.5 / ASP.NET 4.0 bug or limitation (2Gb upload limit). As soon as I constantly get complaints about this from customers.
Are there any bugs records database or limitations descriptions for IIS 7.5 and ASP.NET 4.0 officially supported by Microsoft and publicly available?
Microsoft has one official website where customers can report bugs and suggestions: Microsoft Connect. You can join the site with a Windows Live ID and report a bug. However, I should warn that not all reports are escalated, often times reports will be closed as 'by design' which may not be of much help to you or your customers. But at least the response is official from Microsoft, so you may find some value for not having to bear the burden of proof.
Unfortunately IIS is not listed in the directory of products and programs. In that case your best bet is iis.net as the official source for documentation. I did a search on your particular problem and it turns out it's not so much a bug, but it is a limitation. This particular limitation is documented on MSDN:
Memory Limits for Windows Releases
Edit:
I did find a report on Microsoft Connect about this particular issue, it's in the Visual Studio and .NET Framework site:
Maximum upload size of a file limited to 2GB
Per your comments, we have decided to limit the maxRequestLength value of the httpRuntime element to under 2Gb. Larger uploads will need to be split into multiple payloads in order to be transmitted to the server. Thanks for reporting this issue, Web Platform and Tools team.
It is important to note that though this report is marked "Closed as Fixed" that is because the report is about validating the configuration setting so it can never be larger than 2GB, not because they enabled uploads larger than 2B.