I disagree. If you take the competing clubs from major mfg's there is minimal if any difference when all things are equal. Insignificant. Especially when the lofts are the same and materials are as well. Imagine comparing a 7 iron with a 34 degree loft and one with a 28?Got it. I don't think I agree with the premise that all irons end up resulting the same but if in your testing/experience that's what you have seen, then ok. I know from my experience, there has been a lot of difference in results between types of heads and the vendor line, mainly in the ball speed, spin and height/descent angle departments.
Even if you take an game improvement iron from 10yrs ago and compare it to today it's not that big of a difference.