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I am now convinced that MFI engines need back pressure (whereas low comp CIS motors don't).
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Actually, I don't think that "Backpressure" is the phenonmenon(?) that we're discussing here. If it was it would get stonger and stronger in roughly a linear fashion as the engine is rev'd to red line. I believe that a more accurate description (or mental image) is tuning or harmonics -- as in tuning a piano (but not a fish!). In both situations it's a case of managing the resonances. Adding or removing the SuperTrap plates changes the back-pressure, but not the tuning.
When the muffler and MFI are working correctly, the resonances never quite line up (in fact the best muffler is out-of-tune with the engine) and so for the most part the exhaust goes out the pipe as intended. When you put on the "Sport muffler", I believe that there is a point in the rev range just before the engine comes "on-cam" where the engine (specifically the valve timing) and exhaust are in perfect tune which results in a pressure wave arriving at the exhaust port while the exhaust valve is still open -- the result is that the exhaust is pushed back into the cylinder and contaminating the charge. In the worst case, this occurs during overlap (late in the exhaust valve's open cycle) which can result in the charge being pushed back into the intake tract. I believe that this is the what causes full-race engines on carbs to spit-back in the 3000 - 4000 RPM range when using full-race cams. In any case, the issue goes away at higher rev's because the intake and exhaust charges develop enough inertia to overwhelm the affects of the resonance. This wouldn't be an issue with engines using CIS cams since there is little or no overlap to speak of.
What to do? One: go back to the stock muffler. Alternatively playing around with the header primary length might also have an impact. When I had a FF, I had a set of short (1 or 2 inch) extensions that you could add to header primaries to change the tuning. Adjust the cam timing may also make a change. Finally change the shape of the exhaust system, for example the Phase 9 style exhaust may help -- or may not. I wouldn't be surprised if using stepped headers or having a header primary larger then the exhaust port would also help.
Just some ideas.