My situation is not quite the same as yours: I have a Yamaha CK88 with the USB cable going host-mode to an iPad, running Pianoteq. Pianoteq is reporting 22ms latency but I haven't tried tuning anything - that's out of the box.

Anyway it does work. I get audio from the Pianoteq app on the iPad coming out of the CK88 speakers.

I recently went to buy a piano in a shop on a low budget. I started to hook up my iphone with pianoteq 8 on a Roland FP30x and I could have pianoteq going through the speakers of the piano which was totally awesome.

Now I really don't like the touch and I love yamaha's touch (P-125).
It's advertised with audio through USB (not the P-125 a).

Some users said pianoteq on a PC was totally ok with it but as I recall, it needs Steinberg's drivers to work which I cannot setup on my iphone.

So my question is: is it possible to send pianoteq through a P-125 speakers (with max 1.7ms latency) with an iPhone? If not, does it work better with P-225?
I don't care about local control, I can send a midi signal to turn it off.

Thanks for your help!