Yamaha's dynamic allocation scheme doesn't appear to be as good as Roland or Korgs. I own a Roland XV5080 and Korg Triton Pro and neither of them have issues with dropouts when I play lots of notes with sustain. On the other hand I had an expanded Yamaha 9000 Pro which had over 136 notes of polyphony and I had note dropouts all the time.

The issue with Yamaha is that they often stack 3 or 4 parts to make up a single sound. Many of the sounds on the Yamaha arrangers utilize so many parts that your polyphony is practically nil. If you remove the stacked sounds the keyboard sounds become so thin that there isn't much you can do to still have a great sound and decent polyphony.

Until Yamaha makes an arranger with 256 note polyphony or more, chances are you are going to have drop out issues if you use lots of sustain or do elaborate sequences. I'd actually feel more comfortable using a PA1X Pro with 62 notes of polyphony over a Tyros with 128 notes of polyphony. Of course YMMV greatly.