The long and short answer is:
It depends.
If the app can benefit from SIMD capabilities, it will get a 400% boost for the areas where SIMD is useful.
It the app cannot benefit from SIMD, you won't see a single bit of better performance.
Most apps have segments that would benefit from SIMD, others that would find it useless. So, the long and short of it is, it depends on how many areas in the application will benefit from the SIMD capabilities of Altivec.
