The ff in sufficient indicates a short vowel sound for the "u". The single f in deficient indicates a long vowel sound for the "e". Hence, suff-icient and dee-ficient. At least, that's how we pronounce it where I'm from.

OED, Merriam-Webster, and Collins all show a short "e", as in "delete" or "different", i.e. /dɪ/. I've never heard it pronounced as /diː/. Do you have some kind of southern drawl where you put the stress on the first syllable, like some people do with "defense"?