Mine was off which explains the slow dog acceleration up to 2000rpm and normal acc after, making it a bit dangerous @ intersections, etc. I did the fix below took 15 min to glue on rubber caps with ABRO super glue for plastic and rubber and followed up with some hi temp red silicone afterwards. I advise 24hr curing time before sticking back on. It also took 45 min and some skinned knuckles to put the bar back on. It's a bit tight you can take off the glow plug wiring and work mainly from area between the 2 and 3 swirl valve where there is more room. For now it's staying on will see what happens in the future.
The bar is slightly higher on one side and should be stamped 4 on the black cap for orientation it goes on swirl valve 4. Have fun while cursing and working!!! After 2007 manifold was revised and this issue is a non issue.

http://www.vectra-c.com/forum/showthread.php?t=33511

Excellent video on the subject:

Thanks for the info.

Wouldn't it be a major failure if this thing was broken - i.e. limp-home mode?

Nah, if it is popped off, results are car is quite slow up to 2000rpm, like a very pronounced turbo lag, but is driveable. Reason is the swirl flaps wear out and can move up eff. loosing boost through manifold. Also only n. 3 is actuated and the bar is there to keep them all moving in sync with n.3. Usually no codes pop up with regards to this issue but the difference in acceleration with and without the bar is huge.