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The diesel dave section was the section on Diesel. The backpressure section applies to both.
In the civic it was from a 1.25 to 2.25". The size of the pipe gives you diminishing gains. For example a 2.5" or 3" would probably not have made a significant difference. The muffler is probably a more significant cause but I've never spent time measuring backpressure on just mufflers changes. Do whatever you need to reduce backpressure, whether it's a pipe or a muffler...
The subaru stocker was 2" (I think) and I put on a 2.5". At the time it had a 2.5" engine but the installation of a header did a lot for cylinder balance so its data was really combined gains from both. I do not have specific data on it anymore but I did run it for about 10,000 km with just the exhaust (no headers). From memory I think the headers made as much improvement as the exhaust which tells you that sometimes there is more going on than simple restriction.
I think I posted a lot more on cylinder balance somewhere else. If you get the time do some searching on GAMI injectors as they do a lot of work on cylinder balance for piston (AC) engines.
For the comments on accelerating you may only be accelerating 10% of the time but in city driving it accounts for a significant portion of the fuel used. It is more complex to compare the steady state fuel consumption because you have EGR flow and flow limits that all come into play which is to say you may see few gains on some vehicles but large gains on others where during acceleration you almost always see gains.
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