The term "back pressure" is a long used misleading term. What you're actually tring to achive is optimal velocity in the exhaust to enhance scavenging. Fast moving gasses have momentum and pull on the last bit of exhaust in the cylender as the valve is closing.

So long as there's no sudden and large changes in pipe size it probably doesn't matter that much what comes after the collector. It'll have a small effect on tuning but provided it's not so small it's restricting flow it'll just move the torque peak up or down a small amount. Unless every other part of your induction, timing and cam are highly tuned it wont really matter.