create a custom component to incorporate old features of ice:panelStack but to also include performance optimizations with ability of children to not be included in the server-side component tree and to not continually update a child if content is static. Each panelStack would only show the selected panel, but each child would determine whether it would be included in the server-side tree (facelet="false" would be default so same as regular jsf managed content) for example:-
<ace:panelStack selectedId="pane2">
    <ace:stackPane id="pane1" client="true">
         <!-- some content here-->
    </ace:stackPane>
    <ace:stackPane id="pane2" >
        <!-- some content here but this content would be regular
                jsf handled content and managed by domdiff algorithm
                and rendered flag to either show the content or not-->
    </ace:stackPane>
    <ace:stackPane id="pane3" facelet="true">
         <!-- some content here but this would NOT be part of the server side
              component tree unless it were the selectedId of the parent-->
    </ace:stackPane>
</ace:panelStack>
........
The panelStack would require a proxy so that each child could contain it's own form and the panelStack would always be updated as to which child is selected.
facelet="true" would act same as ui:include, but these components can be nested without worrying about the issues regarding ui:include and jstl tags since the div is always rendered, it's just the content of the divs which would be either included in the component tree (or not if facelet="true").

if you want other content to be rendered, you would nest your panelStacks:-
Modification to this could be having a list of selectedId's instead of a single selectedId, but easiest to implement a singleId to be included than a list of them. (take longer to implement).
for example:-
<ace:panelStack id="stack1" selectedId="pane1">
   <ace:stackPane id="pane1" client="true">
         <!-- some content here-->
    </ace:stackPane>
    <ace:stackPane id="pane2" >
        <!-- some content here but this content would be regular
                jsf handled content and managed by domdiff algorithm
                and rendered flag to either show the content or not-->
    </ace:stackPane>
    <ace:stackPane id="pane3" facelet="true">
          <ace:panelStack id="stack2" seletedId="pane5">
                <ace:stackPane id="pane4" facelet="true">
                       <!-- note this panes content is not in the tree unless pane 3 and
                                pane4 are both selected -->
                </ace:stackPane>
                <ace:stackPane id="pane5" facelet="true">
                       <!-- note this panes content is not in the tree unless pane 3 and
                                pane5 are both selected -->
                </ace:stackPane>
               <ace:stackPane id="pane6">
                     <!-- note this panes content is not in the tree unless pane 3 and
                                is selected even if pane6 is not selected-->
                </ace:stackPane>
         </ace:panelStack>
    </ace:stackPane>
</ace:panelStack>

The client="true" does not have to immediately be part of this component, but might be something to consider down the road....(?). Main difference between this component and ice:panelStack is that child pane's content can be removed from component tree unless it is "selected".