I did the Juice Box Plug and Play with stage 4 selected on my 09 3.5. It is great, still does not have the punch from a full stop a V8 would have but after the first 100 ft. or so it is really quite phenomenal. Had stage 5 selected but had two "limp mode" incidents. My tech confirmed what Jerry Burger suspected, the juice box was causing the waste-gate to stay closed a touch too long causing the fault. No problems since dropping it down to stage 4. Here's a link to a thread I started in Oct.
http://www.xbimmers.com/forums/showthread.php?t=311252