IMHO - the insert inside grills like is in the bottom of the Silverbac , isn't really so much 'dual wall'... I mean in some sense it is, but I believe the main benefit of that is related to if/when your grill overheats or you run it really hot, that insert thing will act like a shield to keep the outer portion of your grill from blistering or discoloring... it doesn't do all that much to provide insulation IMHO.
FYI, Fast Eddy's PG500 has a dual wall around the firebox area ( left hand side, bellow the direct grilling area, right were all the heat comes from )... I'm 99.9999% certain that the reason this area has dual walls, is because if it didn't the exterior of the grill near there would become very very hot and very likely would discolor badly when you run the grill anywhere near it's maximum temperature. However, this dual wall area, probably does very little to improve efficiency of the grill.
Anyway, I don't consider anything less than dual wall throughout the entire grill to be dual walled construction. But that's just me.
As for 430 stainless rusting... cheap stuff sure can, but unless you live like directly on the coast or something, I'd suggest that there's not much wrong with using 430 SS on the exterior of a grill... There may be some good arguments for running 304 SS for some of the internal components , but I see very little good reason to use the more expensive 304 SS everywhere. I've got a old propane grill that's sat outside through about 8 Michigan winters ( spring, summer, and fall too ) and it has only very minimal surface rust that is easily removed with something like Barkeepers friend... So, I don't buy into this idea that if it ain't 304 SS, then it ain't no good for a grill...
Personally, I just simply can't recommend the Firecraft Q450... I may be the only former owner that feels this way about it, but I didn't find it to preform to my satisfaction.