Water is a pretty good idea for testing the weight bearing capacity of the shelf- as long as you use a wide/deep container that spreads the weight out to something close to that 17" x 17" dimension. A tall container will focus more weight on a smaller portion of the shelf and further stress it.
40 lbs., spread out over a wide area, is not that much weight for a typical oven shelf. Think 28ish lb. Thanksgiving turkey + pan + veggies and you're in that ball park. It would be kind of ridiculous for an oven manufacturer to produce a shelf that can't support a big Thanksgiving turkey.
Phil, it's pretty rare that I say this, but there's a chance you might not need 1/2" steel plate. From reading your older posts, it looks like your goal is primarily to reproduce Dewey's pizza at home. Dewey's, being a NY/American hybrid, doesn't need the fast bake times of 1/2" steel. 1/2" steel is only if you have an inclination to make an authentic NY pizza.
What kind of stone are you working with now? I've seen screens in some of your older posts- before you take the steel plate plunge, I'd do two things:
1. Remove the screen from the equation and learn how to master a peel
2. Go to Dewey's, and, if possible, time a bake.
Once I know their bake time, I'll have a much better idea of what stone to recommend. There's a chance you could get away with 3/8" steel plate or possibly even 1/4". There's also a chance you might be able to get away with something as inexpensive as quarry tiles. Maybe. Generally speaking, you want to be able to hit the bottom of the pizza with heat (from the pre-heated plate) WHILE the top is being broiled, not before or after. This means that you want the stone towards the top of the oven, not the bottom. Moving the pizza around mid bake really doesn't work. The biggest downside to 1/2" steel is that it can be difficult to take in and out of the oven so you can bake other things. If there is a chance you can get away with 3/8" or 1/4" steel or even a ceramic option, it will save your back when it's time to use the oven for other stuff.