>Portuguese and Romanian have perhaps already grown about as different as
Celtic and >Latin (and maybe even pre-Grimm pre-Germanic) were 2500-3000
years ago. Linguistic >"speciation" requires a sufficient time depth;
Perhaps, but don't generalise too far... We know the fundamental
difference between Latin, Greek, Hittite, and Sanskrit from before the time
you are talking of. I see no major problem, in the lack of other evidence,
in suggesting that Celtic and pre-Grimm Germanic were equally different.
We can certainly trace within Gothic the relics of verb and noun patterns we
see in Greek and Latin, but we can also see the full-blown presence of forms
not found in those languages, such as the weak decelensions, and the absence
of some other forms (s aorist from memory, though I may wrong on that
example).
Peter