All SC sound engine based Roland keyboards handle polyphonic well..128 polyhony is fine, as most of the early 64 were..

Per arranger the polyphony is handled in this way..Priority goes to drums first (always allowing a mininum of voices), than bass, etc for the most important parts..each with a mininum voice..

The next important way to handle polyphony is last note reserve ..that allows the first note to drop off, making it almost impossible to hear..

I have never heard a drop off on any 128 voice Roland arranger that I have played..

I have had drop offs with Yamaha arrangers (low priced PSR's would mute all sounds till it recovered..what a nightmare).yet a Roland 24 and 28 voice SC board (E-66), would have no noticble drops, thanks to priority voice allocation..

Todays 128 Roland's are more than enough in the arranger line..

I should mention the MediaStation was 256 voice, never dropped out, even though it didn't seem to have the advanced allocation Roland uses...it relied on pure numbers..

G1000 64 voice was fine, as the Korg 62 voice...I recall the 126 voice Yamaha 9000 Pro , did not fare well in layerd pianp/strings...very noticable.(probably poor allocation method)

Another consideration that will save polyphony..Use of sample "parts" (Roland).."elements" (Korg), and whatever Yamaha calls it sound events..

Most of your better sound designers use 1 or 2 sample parts (elements)..keep in mind it will use a polyphony part for each element triggered..The use of velocity switched sounds and velocity filtering save polyphony..Better sound design takes advantage of this..

The key to using less parts/elements are bigger , fuller samples..limiting the need for more partials (elements) needed..
Filtering and changing of the larger samples does not use more polyphony, as added parts and elements do..

Here again is where the Roland G70 or BK series shines, most are 1 or 2 partials (complete samples)..compared to as an example 4 element Korg sounds (use 4 polyphony) for a single sound..
Velocity switching sample tones is a big reason Roland handles polyphony better than it's competition..

Just some things to think about, while you try to understand the polyphony issues..