The ExtPascal version we use in uniGUI has been slightly modified and some of the bugs important for us are fixed. It does not limit us, as Extpascal has a flexible architecture which allow us to import components that doesn't exit in standard Extpascal pack.
Yes, we have created some new components like uniCanvas which doesn't exist in Ext JS. It is possible to create new components from scratch and use them in uniGUI.
OTOH, uniGUI allows client side scripting which enables to have direct control on client side behavior of Ext JS controls. For example see Google Maps demo.
IE6/7/8 doesn't support HTML 5 so canvas will not work.
We will try to support IE6+ but our main focus will be IE8+, FF, Chrome, Opera and Safari.
I believe that it's a futile effort to try to support and fix all issues and aspects related to IE6/7 which are really old browsers. JS engine of those browsers are really slow and not good for complex UI. Not to mention the severe memory leak issues in those browsers.