no car ever needs back pressure... have you tried breathing out of a straw lately? "you need back pressure" is an old myth... people who had no understanding of the engine and exhaust system slapped on the largest exhaust they could find in the past and found out that they lost performance... those same people thought that back pressure is what gave performance because they just swapped in an exhaust with no back pressure... it's NOT that simple
the best exhaust system is always a well engineered size for the best flow rates and minimum back pressure.... reducing back pressure is real easy... tuning flow rates is not.. you have to cause a scavenging effect... on all engines, there are exhaust pulses that occur each time the valves open and closes... the rate of those depends on the number of cylinders you have... what needs to be done is, you have to choose the size that does not cause the exhaust pulses to slam into each other, but instead have it in a smooth continuous sequence... the scavenging effect will happen by using the previous exhaust pulse to "suck" the new exhaust pulse towards the tail pipe... back pressure will cause turbulence because the pulses slam into each other... too big of pipe will cause none of that scavenging effect because they are now too spaced out from each other within the exhaust system... think of it as having exhaust pulse, then nothing for some distance, and another exhaust pulse... that's bad
