I"ve had my gridiron for 6 years. its used an average of 3 times a week and i've never had a problem with the roller grates warping etc. possibly the ones that sagged was an older design.

Are the grates hollow or solid? Mine were hollow tubes, and when you picked up a grate by the rollers, they would flex under the weight of the grate.
 In any event, it should not have happened. I replaced them with 3/8" 304 SS. Problem solved. Forgot to say that others complained about them as well.(I even called Tim about it). Might have been a problem that was solved on newer versions.

So with your finding of a 1/8" plate working, any plans on just building a new diffuser from 1/8" SS?

So with your finding of a 1/8" plate working, any plans on just building a new diffuser from 1/8" SS?

With the warp corrected by bolting on the 1/8 plate, I see no need to build a new d/d. I have seen reports that the 1/8" diffuser  offered in the aftermarket will still warp is some instances. I suspect that it is the altered shape and size of the single piece of SS that causes it to warp. i.e. a flat piece of 1/8 heated to 500 won't warp easily (very stiff). But if you bend and twist it into a different shape, you alter the characteristics of the metal in those areas affected. I am not a metallurgist but do know that various manipulations of certain metals changes their physical properties to some extent. Meanwhile if 1/8" 304 SS is stiff enough to do the job, that is what I would suggest to others, to correct that problem without altering the shape or position of the d/d.  In addition, the change in slope of the plate (see previous comments) to balance the heat distribution does help, but in my opinion, is not at all necessary for good cooking, since most pelletfans have done very well by knowing how to use their pits for best results. I am more of a nit picker than a good chef but that is just me. Is 304 SS necessary? or even 430?. I don't know. Perhaps even just good carbon steel plate would do. I'll leave that for others to decide. I paid $20 for 1/8" 9"x9" plus $10 shipping. You could get it for less locally if available. I also used 1"stainless bolts and nuts to match, (lock washers also).