Sorry, answer to what? The original post is almost three years old, no code was provided, and the sample doesn't exist anymore. Do you have your own sample you can share? And how does the dialog '[render] the widgets off-screen'? GXT supports lazy rendering, hideMode=OFFSETS, but any way that does a render and can measure things must be more expensive than rendering without displaying content.
TabPanel in 2.x has setDeferredRender to ask the content to render as much as possible before drawing anything on the screen - was that tried, or did it not work?