I just got a really beautiful alto G Dixon whistle in the mail, and the thing has a voice like pure gold. It’s just lovely. It’s also very well in tune with itself. However, when i go to tune it, the further out I pull the head, the worse it tune to itself it becomes. By the time I get it in tune with the fiddle, the high G is so flat it’s un-playable. Yes, the fiddle can tune to it, but if I want to play it in a show with my other whistles, it means my dad (on fiddle) would have to re-tune every time I switched instruments. Is there any way to fix the problem, or is it just part of the deal with ‘tunable’ whistles?
There is indeed a limit to how far a tunable whistle can make adjustments. To carry it to an extreme, you couldn’t just pull a D whistle out and adjust it to play in C, for example.
Is just that one high G note out of whack? It might be possible to tune that note.
Are you tuning it to concert pitch? If the fiddle is way off, then you won’t easily find a whistle that will be a good companion.
If you’re sure the hole is flat, and that the fiddle isn’t sharp and that it’s not your blowing, then you can make the hole a little bigger on the fipple end and it will be sharper.
To tune your whistle to a sharp point, I recommend an Opinel #8 carbon steel blade. Just don’t poke your eye out with your new “tactical fipple”. 
Seriously … Assuming no other problems (A440, flat fiddle, etc.), it sounds like you might just be used to blowing too hard, and it’s coming back to bite you. Some of the other Dixons I’ve tried want a fairly gentle breath pressure. If you back off along the breath pressure “plateau”, can you bring the whistle in tune with breath alone, without pulling it out so far?