Hi greg
I'm no expert .
But i think you are on the right track as the bracket they give you only connect to each side wall of each cabnet so by screwing all the cabnets together and then a sheets of melimine internally or one large sheet across the back of all the cabnets you are dispersing the weight evenly .
But this has no bearing on what you are going to screw the cabnets to .So if you remove the gyprock and install a new nogign you can be sure that the internal structure of the wall will hold the cabnets and with some luck the cabnets may even cover the area where the gyprock was removed .
I suppose it all depends on what you are going to put in then ?
good luck
rob
P.s what bench tops are you using i'm about to do my second ikea kitchen reno useing essa stone . looks great and not that pricey if you take into consideration what you save useing ikea cabnets .