There is no doubt that having more UI means more work but you can do an on-demand creation of the panel saving on constructing controls that the user will not get to and also to do external paging. That said the scenario I have created runs very nicely with 600 rows populated with panels but this is meant to test the extreme and that should not be the best practice.
In beta 2 we have done a very massive work on performance and the listview was one of the most inspected controls as with the new UI it allows us to add lots of UI elements without reaching the browser limitations. So I would say that the performance are great and used with on-demand loading and external paging you should not run into performance issues.
As I said we have customers building on the capability and we are starting to get very good feedbacks, so I guess this truly is a very good way to build UIs from the performance perspective as well.