I think your speed spread dependant upon conditions is entirely reasonable and i have also played around with the ZFM and as you say it is interesting to see the yellow bands close.
I want the FMC input data to be as accurate as is reasonably possible.
I am aware of an aircraft that departed with the correct v speeds from an EFB but a -10000kg figure in the FMC which then told them they could go straight to FL410 needless to say despite trying they didn't get there oops
The FMC does consider thrust limited altitude, but the amber band limits do not provide an indication of manoeuvre capability as limited by thrust. The next software release will correct that.
My last comment on the subject.....promise