Many people find that a whistle that is set slightly sharp cuts through the noise of a pub, or session, or a large band - that is why piano accordions are often tuned sharper than A=440 - to A=442 or even up to A=445.

If a whistle is very slightly flat or sharp, a good player can 'think' it into tune. There's a very lengthy explanation why that happens ... this is left to the student to explain this... ;-) {I studied some Tertiary Maths...}

But if the instrument is not set 'true' to itself, it is much more difficult to 'think' every note true, unless you play only the one instrument all the time.

I have dozens of the damn things - some are more suited to certain types of music than others (different tones & timbres, volumes and capability to easily play in different parts of the range) - and if I have not played a particular one for a while, I tend to forget the pattern of 'pushing' the individual notes into tune for each particular whistle.

Loosening the mouthpiece will allow the instrument to be 'flattened' slightly by increasing the overall length, or 'sharped' by decreasing the length, but

1) you may need to cut off a small length of the barrel inside the mouthpiece to get sharp enough

2) if the holes are not correctly spaced, then no matter if you put any one note in tune, you won't get the rest there. For instance, you can't lengthen a D whistle to get a C, or shorten a C to get D - the ratios of the distances between the holes are a constant thing, but the actual distances between the holes vary for each home key pitch of an instrument.

Robin