_mm_unpackhi_epi8 and _mm_unpacklo_epi8 to convert 16 signed chars into 2 signed short vectors

I am using the _mm_unpacklo_epi16 and _mm_unpackhi_epi16 with second argumet vector of 0s to convert signed/unsigned short vectors into 2 signed/unsigned integer vectors. i.e.:

__m128i lowVec  = _mm_unpacklo_epi16(vecA vec0);
__m128i highVec = _mm_unpackhi_epi16(vecA,vec0);

This works fine with 16 unsigned chars vector into 2 unsigned short  vectors using  _mm_unpacklo_epi8 and _mm_unpackhi_epi8, yet when the input vector is of 16 signed chars the 2 short values in result vectors are all 127+original values. 

I found a way to overcome this using add operation with 127, and immediately after the unpack performing substraction of the 127, yet this is very non elegant.

Another way was to use _mm_cvtepi8_epi16 and shift operations to get the wanted values - but this was less elegant than the previous add/sub and the performance was worse.

According the documentation of the _mm_unpacklo_epi8  and _mm_unpackhi_epi8 there was not suppose to be any problem with signed chars...