No idea... Certainly not with any of the 68k family I would wager...
Lookup tables? You could store precomputed patterns of bits against delta values between samples that can be decoded quickly into a chain of +/-128 by simply shifting the bit pattern left and testing the sign.
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I suspect the bigger problem would be computing the required bit patterns as it would could depend on the existing low pass filter characteristics and I bet no two amigas have the same characteristics there.