I'm not sure I'd jump on a 1000x driver. Well ... unless it was given to me or at a very "nice" price. At the moment, the AD900x or A900x drivers run $27 each from A-T. The AD2000 driver runs $45 each. Assuming the 1000x driver falls in the $39 - $40 range, spending an extra $5 or $6 per driver to get the 2000x is a much better choice. If the price difference was large, it might worthy of consideration.
The results are very, very good. With the ATH-AD series at the moment, the AD500x, AD700x and AD900x use the same ear pads. As to the quality of plastic, I'd suspect that the ATH-AD2000x might be a better quality of plastic, but in all honesty, there's nothing to dislike about the ATH-AD500x plastic and ear cushions.
When going for build, consider going to cable that will enter into each cup. I'm not a fan of the wiring used by Audio Technica. It's essentially the same type of "thread" like wire coated in a thin layer of resin to insulate the wire. When working with the "thread" to solder or connect to the drivers, it's a "3 Tylenol" headache you'll be dealing with. I've followed advice here in the forums by others who have dealt with the thin stock wires. You could use some Acetone to strip off the insulated material at the ends of the wires so you could solder them. In such a situation, please use flux, a lower soldering iron temp so that the "thread" doesn't scorch into ashes and tin the "threads" before attempting to solder them to the driver solder tabs. This is why I'd recommend a new cable split going into each cup. The right side plastic is easy to drill into the bottom with a fresh hole. Start with a smaller bit, then bump the size up one or two times until you reached the desired hole size to thread the new wire through. Use shrink tube (a couple of layers) where the wire will feed through the new hole on the right ear cup. It will have an appearance very similar to the stock left side which has a rubber sleeve protecting the left cable - sort of like a strain relief. Also, when working on the recable, use two small cable ties - one for each side so that the wires will not pull out through the plastic cups and wreak havoc at a later time.
The sound when using the AD2000 drivers in the ATH-AD500x is stellar. The soundstage widens quite a bit. I only had 10 hours into mine and could detect a bit of improvement with the sound in the lower frequencies. The instrument separation with the AD2000 drivers is just beautiful. One can build a "Poor man's" ATH-AD2000x in the $200 range. $80 for the ATH-AD500x headphones (new) and $90 + shipping for the AD2000 drivers, plus about $30 in cable supplies.
Enjoy!