Hm, I don´t know with the "stack haste for RPPM" thing. Just sat 2 hrs with my fellow WL (pretty skilled fella) in TS and we analyzed our logs from the past few weeks (We both have Hydra, Wusho and he has Lei Shen trinket aswell).
The result is disillusioning. I compared fights with ~13% haste difference (used to play Hit>Int>Haste 45.9%>Mastery>Crit) now I play full haste to play arcane occasionally.
We compared 29 fights (kills only) in total 13 with 57 - 59% haste and 16 with ~46%
0.3% more uptime for breath with 13% more haste (average)
0.1% less uptime for wushoolay with 13% more haste (average)
Then we compared whole raid nights with boss kills, trys and trash all together and we had:
1.1% more uptime for breath with 13% more haste
0.9% more uptime for wushoolay with 13% more haste
What happend there? You always have a downtime before a fight, so the RPPM multiplier can stack to an amount where you will nearly have a guaranteed proc at the beginning of a fight and the fights are too short to even out RNG. That´s why haste didn´t had the expected influence on my RPPM trinkets in my last boss kills.
This is just our assumption, but you guys could maybe check your logs aswell to compare with our findings.
Btw. a good argument for full haste builds is the significant higher Brain Freeze procrate, higher FoF procrate (there is RNG too, but it is not as bad as RPPM) and higher Bomb damage. Maybe it could be a viable option to play with full haste before having 1 or 2 RPPM trinkets.