The Mercedes EGR valve seems to be different (besides the 90 degree turned fitting). Not shure if that works.
Also it has different part number from Denso - 135200 - 0360
I found an OEM one for 40 euros on ebay Germany:
Porsche 996 986 Boxster Umschaltventil Abschaltventil 99311324501 Control Valve | eBay
Regards, Markus