RE: GridPanel extensibility
The Coolite Toolkit controls render out just standard JavaScript. Adding menu's, toolbars, etc is not a problem even now, although the Ext configuration scripts would have to be generated manually.
Eventually (v1.0), the complete Ext widget library will be supported and available as .NET web controls.
I might be able to come up with a code sample demonstrating how to integrate a Toolkit control with other Ext config widgets/objects. Do you have a specific example of what you are trying to build? maybe with just ext code?
Hope this helps.