The Z stand is very sturdy indeed. Ultra sturdy. I have my P-525 on a Z-stand seen in the pic above. There are external speakers connected to this piano, although I rarely (or pretty much never) use them. The sound and levels coming from the onboard P-525 speakers are very nice. The external speakers were just for testing - to see how loud we could get if we did use speakers. And it can certainly get relatively very loud with the externals hehe. The sounds from the externals are very nice too.
The Z stand (or the one I bought) takes up more space in front of you than X-stand and the stock Yamaha L-515. The more front space taken is no problem for me, but just mentioning it - just in case. The dimensions and build of the Z-stand is what allows it to be very sturdy. Solid stuff.
And upstairs, I have a P-515 on the stock Yamaha "L-515" (self-assemble) stand, which is rock solid. And in another part of the house, I have another P-515 that is on a double-x stand, which seems to be rock solid and not wobbly at all. With the one on the double-x stand, I just use that particular P-515 for standing - without chair.
In your pic, is the base of that piano resting on the rubber sleeve? The sleeves (I think) are meant to provide some cushioning, damping etc.
