Consistent South English would have been "greet feet". Consistent Irish brogue not only would have been but to the best of my knowledge is: "grayt fayt", i e the Irish acknowledge a difference between "feet" and "feat" even in pronunciation.
But older pronunciation was over all something like what the French would have written "graite faite" or "grête fête". And that is the reason why it is spelled with the English spelling for what was that sound: "great feat".
If this spelling is phonetically inadequate, this is not because the spelling is wrong over all for English dialects, it is because only the Irish dialect of English keeps a decent pronunciation: "your spunkier than tay" (as in "than tea", but pronouncing it correctly for once) as goes a song not unknown to the Dubliners.
And as for "right" and "write" to be pronounced the same (as in "I write right") the story is that RIGHT was earlier pronounced as German RECHT, except that English had short I where Gm has short E. Then the GH sound (Gm ICH-Laut) was lost and I lengthened. In WRITE there was a loss of W in WR, a long or lengthened I before the short consonant pertaining entirely to following syllable, then a loss of vowel in that syllable - which lands us with both pronounced REET. Then REET became RYT. Y=as in Fr "œil" (still so Martha's Vineyard) or "ail".
Both inconveniences, espacially the latter, are there in other languages, the more so the older. The former inconvenience is more accented in Irish Gaelic.
Which disposes of this