If that is so, and you can live with it,you will possibly lose some low end torque.You probably will not gain anything from the headers. Other members will chime in but my understanding is, 1 3/8 is the number for a stock motor. If you go bigger you may start to lose low end torque. SF Don?/
Compared to the stahl headers, these ones are shorter before it reaches the collector, so I don't think you can compare the size of primaries based on just the diameter, you should also take lenght into the calculation.
It's about the total volume, but I might be wrong