There is a good article in the May, 2002 issue of Grassroots Motorsports the deals with exhaust tuning and back pressure. I know there are several technical articles on this subject on the web, though I don't have them bookmarked on this computer. Note that the difference between no exhaust and a high performance tuned exhaust can affect HP by only about 8% max. I have driven my 190E 2.3-16 with no exhaust and with its custom stainless system, with and without a resonator and I cannot feel the difference. The differences are so subtle that it only shows up with a dynamometer.
You might consider disconnecting your exhaust, driving the car and see if you feel any difference in performance. You could certainly rule out whether or not the problem is due to a restriction.