Correct, however one would think that the Pacesetter longtube headers that come with a 3" collector, the pacesetter 3" ORY pipe that is meant to go along with the kit, into a 3" cutout & a 3" SI/SO high flow muffler, that the system should have been free'd up enough to get the right amount of flow from it. When I added the cats back in, I opened them up so they were 3" in/out instead of the 2-1/2in 2-1/4out that they were. So in terms of pipe size everything was the same, so no restrictions in pipe diameter that would speed up the velocity, only difference is the slight restriction that the cat's internals made. The restriction is basically the same as someone running a baffled muffler as well (so basically one that you cant stick your arm through like mine). The result of that is pressure being built up in the system, which then increases the velocity. This is also the case when going from a larger pipe to a smaller pipe, yes you gain velocity but in order to gain that velocity pressure must be built up first.
Look at water pipes or even your hose for example. When the water is just coming straight out of the hose, no nossel, there is no restrictions in the system (minus that from the hose itself) so it flows normal and will shoot out 3ft or so, if you add a nossel to the end of the hose (smaller pipe in terms of exhaust) to make the stream more fine the velocity of the water increases dramatically and will shoot 20ft or more. If you feel the hose though, the pressure needed to make that velocity is a good amount more than that of it flowing regularly. Same goes for when you stick you finger in front of the stream coming out of a non-restricted hose (adding a baffled muffler in terms of exhaust) you can make it shoot farther but the pressure builds up in the hose. This is where we get term "back pressure" from.
If you turn the hose on higher (giving the engine some gas) you increase the velocity and flow coming from the source, this also creates pressure. Even with no physical restrictions, the hose (pipe) and any bends creates a pressure build up at the source. So while you are correct, if the pipe size is the right size for the system you dont need any restrictions in the line because it is getting its optimal pressure from the piping itself, the problem is a stock motor wont put out enough to create that optimal pressure with just the piping alone, so to get it, you either need to adjust pipe size or add a way to build up that pressure to make the velocity match the flow rate.
I do a lot of water line designs and the concept is the same.
