Maybe it is less air as compared to the propane units, but I just did a test over 12 hours on the
AMNPS Tray and it would not combust over 300, rather it did well, it lasted over 9 hours with a full tray, though once over 300 the burn times of the pellets were accelerated, at 400 still no burning but as you said they blackened and smoke time for a whole tray was only around 2 1/2 hours but smoke was good, just accelerated burn time, but no flames, even at 400, this did not bother me as I will not be doing much at that high a temp anyway.
I got 9 hours out of the tray loaded and right at 300 degrees, so was very happy with that, but again I am doing this on a electric smoker and air flow is somewhat less than the other grills and even large smokers unless throttled back on low temp with air shutters closed a bit.
One could try to locate the pellet burner low in the box to escape full heat in the upper areas, not sure you have that option looking at that pic, it would seem your best spot would be on the bottom far left or right according to that pic, that is assuming that the burner is center on the bottom.
So take a hard look at the configuration, but as far as the Tray, I have had very good results in time of burn and smoke produced and had no issues, and the smoke was wonderful, I was pleasantly surprised that the pellets produced food that actually had better taste and penetration of flavor than that of my Char Grill with Charcoal or Wood, still trying to figure that out, I think it is the fact that the smoke actually travels over the meat rising whereas the Char Grill it is coming in from the side burner and hanging in the top and coals only produce limited smoke from drippings and do not contribute to the wood flavor.
Anyway, you will love the pellet burners which ever one you choose, I like the tray, and another reason I like the tray is if I want more smoke, I can light both ends and it burned that way for a bit over 5 hours, so I really like the concept and the smoke it produces.
Also, just a note, not disagreeing with DS, just what I found, and the reason may be because my
AMNPS Tray is in the bottom of the box, just above the heating element, and I re-positioned it so it is above the open part of the element, so it is not subject to the intense heat as it would have been in the original spot the chip tray was in, and it is only 5 inches above the air intake hole in the bottom of the box, so this may be why my results were different.
Again try and get the pellets as low in the box as you can, if you can get them beside / below the burner then you should get those same results, and make sure it does not get dripped on, will put it out and create some airborne ash in the box.
You will love the pellet burner though !