Too add confusion to this place, here'* my opinions.
Go to the local strip. Are they out there bolting pipes up or taking them off?
Terms are being missused here on BC a lot.
Backpressure is the amount of restriction in an exhaust. Backpressure doesn't make more torque. Backpressure can only make the gasses exiting the combustion chamber stay there more then not.
Scavenging, or the Scavenging effect, is how a good exhaust system or header design gives the engine more power. Performance exhaust is all about getting the exhaust gasses out from the combustion chamber as quick as possible. The exhaust pipe has VERY little effect on this. All engines benefit from this, highly tuned ones get A LOT.
Header design is the most important part of power in an exaust system. The key to the most power is getting it to exit fast. That usually means keeping it as fast as possible while giving ample room for al the gasses (diamter). The biggest constraint is the collector design, and length of the collector. As long as possible, while having very narow angles between primaries, and flow benched joints.
The exhaust diameter is the same, as long as you can get the air out as quick as possible, it doesn't matter what size it is, but if you make the diameter 3x the size of your collector, what'* the point?
Oh yeah, get some damn headers.