What are the Group Policy settings to control the IE9 “Disable add-ons” toolbar that wants to disable slow add-ons

There is no native way to do this with Group Policy.

Others will come along and suggest you write a login script for the users to set the Site to Zone Assignments locally on the machine which will still allow them to add/change sites. I think that's a really bad idea personally.

We handle this by getting Site/Deskside Support to ascertain the problem, and then submit a ticket to us to add the site. We still retain control, because as professionals we know what's best. Not the users.

Does it create a little extra administrative overhead? Yes. Is it less overhead than all the extra work required to resolve infections and outbreaks in the environment? Yes.

The way we handle that scenario is to move their Test PCs into a Test Lab OU, create and link a GPO (that only sets the Site to Zone Assignment List) specifically to that OU, and delegate Group Policy Object Editing for that GPO to a member of the Dev/QA team.

They can then edit that single GPO to their hearts content.

Disabling the option "Enable third-party browser extensions" under Tools -> Internet Options -> Advanced (or Control Panel -> Internet Options) successfully disables most browser bars while still allowing the typical plugins (Java, Flash, etc).

If I recall correctly you can control this option in GP. Setting can be found under 'User Configuration -> Policies -> Administrative Templates -> Windows Components -> Internet Explorer -> Internet Control Panel -> Advanced Page -> Allow third-party browser extensions'