I don't have a boost gauge, but I noticed a noticeable drop in acceleration in my '98 Passat 1.8TM/GIAC when the DV went bad. Took me less than 15 minutes (my belly pan had already ripped off -- of course) to replace the OEM DV with the TT DV, and voila, my car had its old ripping personality.
Well, when mine went out after I got chipped, I got a surge in power before the turbo engaged. It seemed to accelerate normall up to about 1800 rpms and then stop for a second and surge. It was also really difficult to get a response while driving on the freeway. I'd floor it and usually nothing would happen.