if I need to take a chunk of starmetal and forge it with the breath of Kagutsuchi and then cool it off with the piss of Gaira to create a +25 item, that's more of what I wanted to know.
More this, as it's simply impossible to reach higher than +15 with the 'normal' rules for forging items. Needs crazy-special ingredients, crazy-special forging techniques, and likely special circumstances like 'the stars are aligned correctly' or 'you have a god working with you'. It would probably be a huge quest at the least to even
find out how, possibly a lifetime goal of the character to actually accomplish. The sort of thing that would make a good purpose for an Ebudan, for example.
Basically it comes down to 'GM decides, because it isn't covered by the rules.' :)