Yes, I have successfully modified the TCP idle timeout numerous times, as I'm sure many others have.
What is the exact error you are getting?
Yes, we must increase the HTTP idle timeout for our online application to 10 minutes. I had no problem doing this. I have a custom Generic App Defense for the rules for this app, and I simply changed the HTTP TCP Idle time to 600. We're on 8.2.1, but it worked fine when we first went to 8.0, too.