If you really want more power, your best bet is with SpeedtuningUSA, one of there chips. The way the finns did it was take the 606 turbo engine and transplant it into a 95 w124, with the 603 mechanical injection pump. To make a super diesel in the W210 body would be get a custom ecu program or to get rid of it and make it all mechanical. Check out this site for more info on it.
http://mersuforum.net/forum/viewforum.php?f=29
Oh yea, also you can only make so much more power with the stock turbo. The most boost you crank out of it, the more heat it produces and you will eventually melt something, as the finns already have.