Tom, I'm going to have to disagree with you on this one. Back pressure is a bad thing regardless of the engine make (talking about 4 stroke engines only). The engine has to do work to move the exhaust through the pipes. Extra resistance means less power and less fuel mileage. The engine is an air pump; more air in and out means more power. Even with 2 strokes, a lot of people confuse back pressure with proper tuning of the sizes and lengths of the pipes.

A good exhaust system will improve the scavenging and can quite effectively change what the engine wants for jetting. I suspect that your car might have been running on the lean limit and when you changed from the restricted exhaust to a freer flowing one that it went over the edge. Three or four jet sizes up usually cures the problem. I'd really advise trying it again, there's power to be gained!

As with any major change, cam, air cleaner, heads, intake manifold, carb, you have to start with a clean slate and tune the car to what it wants after the change. Sometimes it isn't much of a change but make sure anyways. Adjust the fuel mixture with the jets in the carb, not by restricting the exhaust. Would you install a smaller air filter to richen up the mixture? Some people do!

This is from many years of experience solving other people's tuning problems for the street and the track. Also, most dyno operators don't tune the low to mid throttle and rpm ranges and leave that up to the owner. Most dyno tuning is done at full throttle and higher rpm; I tuned the engines for proper street manners after the engine was installed. Been there, done a lot of that!

Build an unrestrictive but not unmuffled exhaust system and tune for it.