Back pressure is ALWAYS bad (except when dynamically controlled, see bottom example)!
If increasing back pressure add HP, the headers primaries are not matched to the engine.. ( probably too big in diameter).
Primaries diameter must prevent velocity loss.
Primaries and secondaries length will affect power band (scavenging effect boosted by a strong backpulse created at their end ). A short length will boost the top end power, a long length will boost the midrange or low end power.
When using full straigthru without X or H pipe, the secondaries length will be equal to the total exhaust line, it will boost low-midrange power, not topend.
When using X or H pipe, a back pulse is created shorter in the exhaust line, it will be better for high and top end power. And you can tune the peak boost by moving the crossover location.
When using stock Vmax mufflers, a backpulse is created in the first chamber => sooner than in some straigthru aftermaket exhausts. That's why max power can be better with stock exhaust !! (but low-mid power can be lower)
Big diameter primaries will kill power everywhere, except (may be) at high rpms, backpulses created by secondaries won't correct the loss and creating back pressure will be the only way to counterbalance this. So this is just a bandaid...
Exup system are made to use big diameter primaries (Vmax 1700). Top end power is very good, because big diameter primaries are used, low-mid power is preserved by backpressure (Valve partially closed)