The ability to modify the pitch and tone of any mouth-blown instrument by providing a "tuned mouth" is a part of learning any common band or orchestra wind instrument. One might say that learning this bit is sort of the demarcation point between being a "kid in the band" and a potential musician.

In whistle, proper tuning of the "back resonance" can be a very great assistance in flipping between the registers/octaves, but for some reason whistlers seem not generally to recognize that they do it. My observation would be that mostly they do - they just aren't really aware of how/what makes things happen. Perhaps a well-learned skill doesn't really need to be analyzed too much.

Based on my "unpracticed" experience with (mostly Generation) whistles, I'd expect a player could "pull" an individual note by about a quarter tone for experimental purposes, but perhaps half that would be a reasonable adjustment in actual performance(?). (Pulling a tone down seems a bit easier for me than going up.) If you don't "think" the right pitch you probably won't get it dead on, no matter how perfect the whistle.

Which of course doesn't suggest that a whistle with a plug in it doesn't perhaps merit a bit of work...

John