Chemical engineer by education, Process engineer in a steel mill by trade (we manufacture electrical steel).

There are no metallurgical differences nor chemical differences to my knowledge. I work in decarburization (carbon removal) and finishing(we anneal then coat the steel in a non conductive coating), but talk to the metallurgists who control the heat chemistry and what they DO care about are

-Carbon

-Manganese

-Phosphorous

-Silicon

-Chromium

-Nickel

-molybdenum

-Titanium

-Copper

-Tin

-Aluminum

-Nitrogen

-Oxygen

-Lead

-Boron

Some of these are only present up to ~3% while others are on the orders of PPM (parts per million) or PPB (parts per billion).

To the original question, any one of these elements being out of wack can ruin the heat and make it a different grade of steel or completely ruin its properties. I have never heard of Iron 54 or 56 ever even mentioned regarding melt chemistries. So I would say this supports a lack of difference both chemically and metalurgically between the isotopes