I too love whistles (of any size) that have sweet high notes.
The size I have the most experience with is Low D. I must have tried and owned around 30 of those over a period of a few years when I was buying, trying, and selling Low D’s in search of the Holy Grail.
It appears that the hardest note to get sweet is High B.
Unlike flute, with Low Whistle the voicing is built-in and can’t be changed as you play, so makers have to find a compromise between making the high notes (especially High B) not too loud and harsh, and not having the low notes (especially Low E) too easy and quiet.
The lower notes of the low octave and the higher notes of the 2nd octave are never going to be as balanced as they are on flute. There’s always going to be a volume differential.
Thus the Low D’s with the nicest sweetest high notes, like the conical-bore all-plastic Tony Dixon Low D, have low notes which IMHO are too soft to be useable in a group setting. The Low D’s with a huge fat low range are going to have loud, and sometimes harsh, high notes.
This really hit home when I was playing a Low D along with three flute players. On High B I was louder than all three flutes together, on the low end of the low octave I couldn’t be heard at all.
One of the most balanced Low D’s I tried was the MK. It’s partially due, I think, to the voicing: to play the octaves in tune you have to blow the low octave strongly and blow the 2nd octave more softly than with most Low Whistles. In other words the difference in blowing pressure between the octaves is quite small, which makes the 2nd octave feel easy and effortless.
But for overall performance I’ve settled on Colin Goldie whistles. I have great-playing Goldies in Low C, Low D, and mezzo/alto F.
Colin makes his whistles with a variety of windway heights, and my Low D head with the wider windway gives fatter low notes and sweeter high notes, though at the expense of some air-efficiency.
For mezzo/alto A I use an A body on a Jerry Freeman Generation Bb head.