The top engine in a Sprinter is only about 140BHP, so I doubt they'd get far beyond 100mph for something with that much air resistance. For comparison, a Zafira (obviously much smaller and less air resistance) with similar power tops out at about 120mph.

If you want to see a really fast van, how about one of these:

www.vitosport.co.uk/vanx.php#engines

I've never seen one here, but they are very popular with couriers in southern Germany, where they frequently get used to their full potential on the Autobahns. Last time I was there, one went flying past me like I was stationary ... I was almost flat out in an Astra diesel doing about 160kph at the time!