I have the 3.5" downpipe to 4" exhaust system. It definitely made a difference. I also wrapped the the pre-turbo pipes, downpipe and exhaust past the transmission. My van has the gauges (exhaust temp, tans temp and boost guages). I found the trans temp didn't change much from the factory exhaust, but it still ran in the 180-210 degree range on longer runs. Then I added a Banks Transcommand. That helped drop the trans temps a little, but the shift patterns were not very good (I did not like it at all!) Then I got a custom programmed Bill's Chip with 4 different programs plus stock settings and removed the Transcommand. That made a huge difference. I've found it gave a big boost in engine power, it reduced the exhaust temps even more, the transmission downshifts way less on the freeway, the shift patterns are solid and predictable, the torgue converter holds tighter and the trans temps now run much cooler, especially going up/down long grades or towing the trailer. It also improved my gas mileage on the freeway. It does smoke a little more when i put my foot down and I have to keep an eye on the turbo boost as it can create over-boost situations.
In summary - if you have the exhaust and the guages, you'll want the chip! Bill's work can now be found at:
http://www.gopowerhungry.com - check out the Phoenix 6 position chip. You can use his programs and/or create your own. It's the way to go.
-Marc
2000 EB50 Q4x4 PSD MTN VAN