Running open headers doesn't make the engine weaker. It moves the peak torque move up to a place on the rpm range where it's hard to use on a street bike.
Open headers are referred to as Drag Pipes, This is because they give you max power at a very high rpm like you use for drag racing.
You can adjust the length and diameter of the head pipe to get the max power at most any rpm you want. It is a very narrow range of rpms. +or- a few hundred rpms. Out side of that ideal rpms the power drops off drastically. For the street well tuned exhaust gives you lots of power over a wide range of rpms.
Back pressure isn't the right term to use. Yes, a good exhaust does create some back pressure. Just adding back pressure to a poor exhaust won't make it a good exhaust.
I can't explain all the theory on exhaust. I know a bit about it.
At high rpms an open exhaust will work well because the airflow thru the engine is giving max cylinder fill. As the rpms come down to the mid and lower ranges the air flow thru the engine is less and it's harder to get max cylinder fill.
At the lower rpms exhaust gasses travel in pressure waves down the headpipes. At these rpms with an open header tuned to high rpms these pressure waves reflect off the ends of the pipes and return up the pipe and get to the valves when they are open, this lets exhaust get into the cylinder. This exhaust revrsion can delute the chrge of air/fuel in the cylinder and if strong enough can push the incoming a/f back out of the cylinder. This prevents getting the max amount of fresh a/f into the cylinder. Less, a/f less power.
On a headpipe tuned more for the mid to lower rpms will give more power because the time it takes the exhaust pressure waves to travel down and back in the header is adjusted to get to the valves when closed. this prevent the exhaust from getting into the cylinder.
When the exhaust gasses go out the headpipes in pressure pluses there is a negitive pressure pulse between the positive pressure pulses. This negitive pressure helps pull the next pressure pluse along. This is scavanging.
Scavanging helps pull the spent exhaust out of the cylinder and fresh A/F in.
All these things come into play while designing headpipes. They are a bit more than just a way to get the exhaust out of the engine.
The mufflers need to not slow these exhaust gasses from slowing don too much. They do this by opening up to a larger diameter so they can slow down without slowing the head pipe flow. This slowing effect quites the exhaust. A muffler needs to do more than that to be quite. They often have baffles that redirect the flow to further slow the gasses. They can also use a sound absorbing insulation to help quite thing down.
I hope this helps understand things better. If you want to know more I might suggest googling Exhaust Systems and researching more on it.
