Hello,
One of the changes introduced by FastCGI ISAPI 1.5 Beta is the removal of the response header validation. According to http://blogs.iis.net/ksingla/archive/2009/04/20/fastcgi-isapi-1-5-beta-for-winxp-and-win2k3.aspx, invalid response headers are now silently dropped from the response. I don't know why other customers have complained this feature, but I think this feature helps developers ensure all response headers sent are well-formed. I suggest adding this feature back and to provide an option to disable it instead of the complete removal.