When the PS cars, (not to mention others) started using a O2 sensor in each tube, (8 of them) is when we started running into some interesting things. It seemed that as the choke diameter on the collector was tested smaller and smaller there came a point that the O2 readings would get rich for no other reason. If the motor was leaned down, the O2 would still read rich and the car would slow down. Open up the collector diameter and things would be happy again.
Now some might think differently but my and others conclusion is that as the header was pulling harder on the engine with the smaller choke, more unburnt fuel and air with it were being pulled out of the engine instead of in the cylinders. Many have learned that this points to power left on the table as a cam timing change and other changes can utilize the increased depression on the exhaust side of the motor to make even more power with the smaller collector diameter. Not having O2 sensors in each tube, I do not know how else to determine what I call overscavaging when in the car.
Another point is if your header is too large or too long, it is also possible to hurt power straight out with too small a choke.
And yes, you do have to have extensions and adjustable choke collectors or several on hand and bring them into your testing loops. And no this is not easy, but it is real.
You can just have a bigger header and not worry about it all...
And some other thoughts;
A given engine wants a certain size collector diameter on a given day.
Higher altitudes can use a smaller diameter right off the bat.
A given diameter collector needs to be a certain distance from the cylinders on a given day. (that may seem to contradict the first sentence, but it does not, a larger diameter may want to be further away.)
If you keep adjusting these variables, you will find a pattern your engine likes and needs for the different conditions you run into.
If you change other things on your engine, don't neglect the above adjustments.
And I think I stated earlier, if your motor is not sensitive to these things then a larger adjustment by me may be in order. I often do that gratis too for my customers.