Well in answer to my question about the stock muffler restricting the flow of my 2000 2.7L boxster with TTP headers and high-flow cats, the answer is that it does not appear to, at least not based on a couple dyno runs. The first dyno run with TTP headers/100-cell/stock muffler was 201 hp. By adding a Dansk Sport muffler to my system the car dynos at 201 hp, and maybe a few ft/lbs less torque, so little or no difference, at least at its current state of ECU tune.
In response to the question about CEL with header and 100 cell cat, yes I am getting a 1275 CEL code. In my setup I put on the TTP headers which have no cats on them or bungs for O2 sensors. The 100-cell cats I use are located downstream near the muffler and have bungs for O2 sensors right in front of and behind the cat. I put on 4 new O2 sensors (with longer cables to reach) to the sensor bungs in front of and behind the cat.
I think the problem in my case is that the cat and the O2 sensors are located too far downstream in the exhaust flow. Because of that the first set of O2 sensors which is normally right at the base of the headers is too far away and is not heating up enough.
I am in the experimental sorta mode so next week I am going to put a O2 sensor bung for the first set of cats at the base of the TTP header, where it is found in stock cars and see if that helps get rid of the CEL.
BTW, as I said previously when I put on a Gemballa muffler, the car was loud.... too loud for my tastes, so I put the stock muffler back on. Last week I installed a Dansk Sport muffler and it is also loud, not as loud as the Gemballa, but still loud. The Dansk also has a 2-3K resonsance that is annoying when cruising at highway speeds.
The reason I tried the Dansk muffler in the first place was to see if I could squeek a little more performance out of the exhaust, not more sound. I will try this setup for a while and decide if I will keep it or go back to the stock muffler. The only plus so far for the Dansk muffler is that it sounds nice outside of its annoying low-rpm resonance range. Also, it's lighter than the stock muffler. How do you say "live and learn" or it it " I know just enough about this stuff to be dangerous....".
Ed
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Ed, did you buy the Dansk exhaust with the "super sound tips" shown below, or the one that looks like a hot dog shape? I have the super sound tip version, and since the muffler itself is shaped exactly like the OEM muffler, there is no resonance at all, just a throatier exhaust note due to the different baffles inside. The hot dog muffler from Dansk does exhibit resonance in the range you speak of.
