Be sure that the retrofit was done correctly on the car. I recently had my 87 300 SDL retrofitted and was done incorrectly. I was lucky that all the freon leaked out of the car. The next shop that worked on it did it right.
My opinion on the 300 SEL, being an owner of a 90 300 SEL, I'm not that happy with the car compared to the SDL. Gas milage is poor, I guess I would say that the overall performance compared to the SDL in my opinion is poor.