It's not quite so simple as a pure gain or pure loss. Here's my graph of average runs (3 runs per configuration, toss top and bottom):
The red line is the baseline, which is with the Dixis section 2 & 3.
The blue line is with the Eisenmann section 1 (secondary cat delete) added.
The purple line is with BMC filters added on top of that.
As you can see, from the addition of the Eisenmann, there was a very slight gain up 3500rpm, then a slight loss for 1300 rpm, then a gain up to near redline.
Overall, it's definitely a gain, no question about it. The total area under the power curve is larger after the secondary cat delete.
I think it's important to remember basic exhaust theory: unless there are some serious design flaws or the engine is unable to adjust fuel delivery, backpressure always reduces total power. This is a point that's often confused. All major exhaust manufacturers agree on this point. Basic info:
http://www.magnaflow.com/05news/magazine/05sportc.asp
BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Here's another way to put it:
http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm
Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.
At low RPMs, you need exhaust VELOCITY. At high RPMs, you need exhaust VOLUME.
The best way to do this is to size your exhaust piping properly, and to use the lowest possible backpressure canisters. It's the velocity that helps scavaging: higher the velocity, greater the vacuum.
Backpressure is a shortcut to velocity, but not a great one, as it hampers power through most of the powerband.
Proper exhaust piping sizing + proper muffler design is the best way to maximize total power.