SessionTimeout Parameter

I was reading that there is a parameter called 'SessionTimeoutWarning' that can be set in eapps.cfg, where a warning is popped up before the session times out to let the user choose whether they would like to continue their session or not.  I also read that it only works for SI applications.  Is there a similar parameter for HI applications?  We are using Siebel Sales. 

Also if not, can we set the SessionTimeout parameter to 0 so that it is not enabled?  What type of impact would this have on performance?  They would either like to have a warning popped up, increase the session timeout (as long as it doesn't cause performance issues), or disable it somehow.