I believe that 1.5 pipes don't need much help, 1.75 are another story.
They have too large a volume. This decreases the exhaust velocity.
The 1.5 is about right as far as diameter. I have no hard proof of this, but most believe the 1.5 pipe is about right for head pipes.
The 1.75 inch pipes need help, this is so because of the inserts that go between the head and pipes as well as between the pipes and mufflers, they sell well.
These may work the same as the inserts by the head.
Leo