I have a project that uses something like 22 tracks to create an animated video wall, and it definitely produces a more useful preview in V5, even when sent to an external monitor.
It shaves seven seconds off my RenderTest run time, too. That's a pretty decent speedup.
Yes - Using exrernal monitor AND Preview with scrubbing frqame by frame the motion appears smoother and the picture looks sharper . . . the sharper bit most be wishful thinking oin my part . .BUT I definately do feel the external Preview seems smoother and more responsive.
This is from my "this is an amusing thing Grazie would appreciate" file.
I rendered my first project in Vegas 5, a 32 minute file being rendered to WMV 9 at 512K. I went away to install Vegas on my laptop and when I cam back the project was done rendering in 90 seconds. I thought, wow this is an upgrade! until I realized I had left "render loop region only" checked.