The advantage for adjust_strategy=2 with integer I seems to get greater for larger jobs. I don't know why this happens, but it does. So if you're doing a big job then you definitely will want strategy 2: saving 5% of sieving time on a c180 GNFS is absolutely worth it.
I don't know how your timings for GNFS and other SNFS jobs compare, so I can't tell what counts as slow. If you're meaning that these SNFS jobs take longer than the "rules of thumb" for SNFSGNFS conversion indicate, then yes  that's because of the large c5 coefficient.
There is no difference between the SNFS and GNFS algorithms once we've got a polynomial. SNFS just means the special form of the number allows a polynomial to be easily found, usually with small coefficients. (Historical note: in the early days, there was more of a difference, because SNFS was initially only developed for numbers of the form b^n + c where the algebraic number theory is easier.)
