I thought I might chime in to give a little more info for early 3.4 NA owners. As has already been mentioned, there is no such thing as a requirement for backpressure. If you look at motorsports where there is no noise restriction, such as drag racing, frequently open stubs are used because these offer the least resistance (the least backpressure if you will). The thing with exhausts on four stroke engines is that they need to be designed to give the greatest exhaust velocity. The faster you can get the exhaust gasses out of the engine, the more power you will make.
In order to achieve this an exhaust needs to be designed with a diameter that is sympathetic to the volume of gas produced. If the bore is too big then you will actually slow down the gasses in much the same way that a river slows down when it widens. Obviously, at different revs then engine produces different volumes of gas, so the ideal exhaust would vary in diameter in line with revs. Until this fantasy pipe is invented we need to settle on a compromise diameter of pipe.
If you are going catless, you will see bigger gains than going with 200 cell cats all across the rev range. This is not just because the cat matrix isn't in the way, but also because the bulb in the exhaust that holds the cat takes energy (velocity) out of the gas by slowing it down where it widens at the start of the cat and then another restriction is faced at the other side of the cat where it tapers back to the normal exhaust diameter.
To get maximum power out of a 996 NA exhaust, the mufflers would also be changed to something that used a straight through design, such as the After Hours one:
https://www.6speedonline.com/forums...ted/232933-996-after-hours-sport-exhaust.html
The only problem with this is the very tight radius of the bend between the mufflers. Although the design is much less restrictive than the normal 996 muffler, more power could be released by increasing that radius.
I'm currently toying with getting something made in the after hours style, but replacing the first muffler with a 200 cell cat, and running the manifold direct to the cat & muffler on it's own side. If I can get the radius of the bends in the exhaust large enough (basically 6" or greater) then I could lose a collossal amount of weight from the rear of the car, and gain somewhere in the region of 15-20 hp.
As regards lamda sensors, I strongly advise you run them. Without the fine tuning of the mixture that they provide you may have trouble with rough running. I'm speaking from recent personal experience with my early 3.4 car here. Even with a decat pipe, I would still keep the lamda sensors. With one of my lamda sensors out of action recently, the bank it was on failed the emissions test for the MOT, and the car only passed because the tester then tested the bank with the working lamda sensor.