Howdy from BV county Mike. I have used the Walmart flats many times while practicing for comps. They are ok with fat on them but in my opinion not enough for high heat. I prefer to do mine at 250* and wrap "around"165* internal. I am not exact with this because I usually am going more for time than temp on both my flats and my packers. Another suggestion I would make is to inject the WM flats with something especially beef broth. I say this only because while they are ok pieces of meat, they are not the highest quality(best we can get in storm Lake though) and with the trimmed fat, these flats have a tendency to dry up some.
There are a few members from the Storm Lake area on here, though with comps going on some of us are hit and miss I myself will be in Emmetsburg today and tomorrow, get to know us and any time you have questions I bet a couple of us have tried stuff at least once.