Much of the above is incorrect. Firstly, you must not have looked at many steel stands is you have never seen one with centre unsupported. I have yet to see one that does.That includes ones I have fabricated myself. Steel is seldom straight. Even square tubing, which is generally better than structural members such as angle iron and channel iron, is rarely straight for any length. In addition, welding deforms steel. I have been fabricating with steel for 25 years, and have had to deal with this in every case.
Not all commercial wooden tank stands are open on the top. If we accept that MDF or HDF can be classed as wood products, they generally have a solid top. A solid top also strengthens the stand strucrure, whether it is necessary to carry the weight or not.
The purpose of styro is not to aid in leveling the tank, only for absorbing discrepencies in flatness. Contrary to what you are stating, the styro will compress until the entire edge is supported. I have 4 frameless tanks (30gals) on steel stands, and the styro (3/4") has compressed on the ends where necessary and the entire bottom edge is supported. You do not want the styro to flex.
Shimming the stand has nothing to do with making the top edge or plate flat, only in making it level. Styro will not make tank, on an unleveled stand, level. That is not it's purpose. It is only there to absorb discrepencies in flatness, and weight distribution. A tank could probably be supported only on the ends if the ends were level and there was no flexing. Still, it would be bad practice.
You are entitled to your opinion, but please, refrain from stating an opinion as fact. Much of what you stated is untrue.