Hey there,
First off....displacement is not changed by simply swapping cams. I wish it was....hehe. Increasing Displacement is done by boring out and stroking out the cylinder heads. This requires new pistons, gaskets, machine work, etc. etc. I did turbocharge my 16V, but I honestly don't know if they make a kit for the 2.6. I also don't know if the 300E 2.6 motor is the same as the 190E 2.6. You should contact them. They have a phone number on thier website.
http://www.mosselmanturbo.com Now if you had a 16V, I'd be able to help you out. hehe, Good luck.