in everything ive ever owned or been taught about pretty much says the faster the exhaust gets out the better, the less restrictive the better.
You're talking about two different things here. Think about putting your thumb over the end of a water hose. Does the water come out of a smaller opening at a higher velocity? Of course. Although the smaller opening is more restrictive
than the full opening, thus increasing
back pressure and velocity.
Conventional wisdom has always been to go conservative on tube size in small displacement applications at comparatively low RPMs. This may not make the best HP numbers, but it will optimize the torque
generated (which is what we want for off road, and for seat-of-the-pants impressions).
You're not looking to move a huge volume of gases. You're looking to move the gases out of the combustion chamber as quickly and efficiently as possible, creating a low pressure chamber in the cylinder during the valve overlap so a more dense fuel/air charge is drawn into the combustion chamber. Pulse timing in the exhaust stream is very important to that efficiency.
Equalizing the lengths might have to do with emissions.
When tuning an exhaust pipe the length is so determined that the sonic waves that develop in the pipe help scavenge the burnt gases from the cylinder.
This reduces NOX emissions and increases power.
So it comes to reason that equal lengths of pipe yield best results.
This is what I'm talking about. That pulse timing aids in an efficient burn. That improves torque, emissions, and MPGs. IMO, the last two were probably the real motivations for the visually funky design.
Interestingly, Magnaflow and AFE have both come to the market with replacement down tubes that are straight. But unless I am imagining something, they have increased
the tube size to slow down
the gas evacuation in that section of the system, "tricking" the system into functioning like a longer, but smaller, tube. By slowing the gases in a shorter tube, they bring the exhaust pulses back into correct timing and maintain the efficiency of the original design.