I guess it's a matter of how someone defines film speed. To me, you are talking about exposure index which can be interpreted as a personal choice for the speed setting and not about the actual film speed. I also assume the reason why you set your EI to a lower than indicated speed setting is to give you a safety factor. I can't see you actually printing the shadow up around Zone IV. IMO, simply changing the EI doesn't really change the Zone placement. The relationship between the tones don't change. They are simply pushed up the curve. Then the higher densities of the negative are printing down. My idea of the Zone System is to visualize the tones of the final print when shooting, so when you "place" a value at a particular Zone, it's not a particular density on the negative, but a relative value on the finished print.
I've attached two three quadrant reproduction curves that illustrates difference between "normal" exposure" (Graph 5) and giving the film a little extra exposure and printing down the additional density (Graph 6). As you can see, there is very little difference.
So, there's nothing wrong with what you are doing. I was just commenting more on the connotations of certain terms.