The 2.25" single dimension primary will not behave similar to a step pipe of any size. Your previous entry noted that you were using a 2.25 primary with good results, and that was the basis on my recommending that you use that size pipe. In that same entry concerning the 2.25" pipe, you indicated that you might try a 2.5" primary which is too big for anything I have ever seen that was using carb's and nature to fill the engine. When you look at all the different combinations that people try to run and notice that you have a combination now and then that seems to defy physics by using an arrangement that should not work well, like the zoomies on the alcohol car using Nature to fill it, (No blower). There is more to the picture than you could see, or there were huge concessions made in the torque department and engine harmony for a theoretical perfect top end tune-up. Let me know if you ever figure out the answer to that one, I am sure it would be very interesting to hear the reasoning. I have had no success nor have I heard of any one having any luck with that configuration on a naturally aspirated engine.
This will be my last entry on this thread, I hope you find this information entertaining or helpful, good luck