No car 'needs' backpressure. That's a myth.
Most people confuse it with exhaust gas velocity. The reason you have a header, with all of its bends, instead of just straight piping, is for exhaust scavenging. What this does is times the 4 exhaust gas pulses from the engine, so that when they hit the collector, they come out evenly... as each pulse come out, it creates a little suction behind it, making it easier for the next pulse to come out of the manifold, creating more power.
Think of it along the lines of how truckers, or race car drivers, 'draft' one another to cut down on wind resistance.
As for the backpressure/exhaust velocity thing, if you have too large a diameter of exhaust piping, at lower RPMs the exhaust pulses will move too slowly to create that scavenging (drafting) effect. For instance, if the car 'needed' backpressure, then you could just stick a huge 3" exhaust on it, but have a restrictive muffler. In reality, this would simply make no power.... instead, most aftermarket Focus exhausts are a decent 2 1/4 diameter tubing (2.5" on SVT and higher power models) with a straight-through silencer.
Typically, a 4-2-1 will retain a little bottom-end power, and be a little quieter than a 4-1. The 4-1 will make more mid-range and top-end power, however.
I hope this in some way makes sense.... i've just woken up, and have had no coffee. I've discussed this in detail on other boards i've been on with my past vehicles (try telling a 4x4 guy that his 4" exhaust won't make any power, even though its really loud!!)
