No back pressure will increase torque across the whole range...
Try it seriously...
Get on a dyno, do a power run or two.
Then go, take your exhaust off, dial in a slight bit more fuel, and nock a touch more ignition advance in.
You'll notice torque jump across the whole range...
And torque is directly relational to power.
And no, i don't have a 3 inch exhaust.
Going too large of an exhaust is actually bad also, you'll increase back pressure again.
But, I hear you all cry, "That's impossible! It's a larger pipe!"
The problem being, in a larger pipe, back pressure is actually increased as since there is a larger volume to fill, and only the same amount being pumped out the exhaust, the actual velocity of the gas travelling down the pipe is slower.
By the time the gases hit the end of the exhaust, they've cooled down that much, they've condensed, and slowed down even more as that same molar volume of gas, no longer fills the same physical volume.
And your power band, is effectively where you have the best amount of torque (Since power = RPM*Torque/a constant) (Constant changes depending on units used for torque and power)
If you put on such an exhaust which you claim is bad, then go and get a tune, and put a touch more fuel in down low, and a touch more ignition advance in, you'll gain your "missing" torque back also.
When exhaust gases are suddenly allowed to flow more freely then they were tuned for, the tune leans out, and needs either: More ignition advance, or more fuel, or BOTH.
Yes, this post is a bit jumping and rambled... I'm tired...