Darkwood Core and Heavy Plating give the same AC at level 17 (22), while Composite Plating is the worse option to choose, getting an AC of 21 at level 17. That is, of course, if the Darkwood Core character has a 20 for Dex, which by level 17 probably would.
Removing the proficiency bonus, or even just giving them 1/2 proficiency would make the warforged rather "useless", as Integrated Protection is practically all they get. It's the biggest selling point of the race.
Taking away the proficiency bonus gives an AC of 16 (11 + 5 Dex), 15 (13 + 2 Dex), and 16 (no Dex to AC). A human in Studded would have AC of 17 (13 + 5 Dex), half-plate AC 17 (15 + 2 Dex), or full plate 18. That is not counting any magical armors.
Giving half proficiency would be 19 (11 + 5 Dex + 3 prof), 18 (13 + 2 Dex + 3 prof), or 19 (16 + 3 prof), which is like +2 studded leather, +1 half-plate, or +1 full plate. Slightly better, but still worse when +3 magic armor come into play.
If you wish to change it, I would give their Integrated Protection equal to the best armor of the tier (studded leather, half-plate, full plate), and create some kind of warforged only magic item that would give them something equal to the various magical armors (+1-3, armor of resistance, etc). Or, allow their bodies to be enchantable as if they were armor (which is what I am doing in my personal game, along with the Tortle's shell acting the same, though still only giving them 17 AC as they have the one ability to increase their AC by 4 when they enter the shell).