That lowes thing looks a bit shakey to me. Not offense to the person using them. How are they working for you and are they sturdier than they look on the photo from Lowes??
I have something like that which I use for some of my other hobbies and they are very strong, very solid & stable. I think putting ~400 lbs on a shelf might be asking for trouble but if you were setting up your tanks for display instead of density (and thus putting fewer tanks per shelf) they would likely work fine.
I've got around 250 pounds of computer equipment on just one of the shelves in my rack and don't even sweat it. That rack is loaded top to bottom with computer hardware, and not home PC's either but heavy old corporate UNIX systems. Sun makes much denser equipment than Dell.
The nice thing about these racks vs. custom wood racks is that if you get frustrated about the amount of space between the top of your tank and the next shelf above, you can fix it. With custom wood racks, you're kind of stuck with what you've got.