It is hard to see how the OFF/ON runtime compare on a log2 scale. Here's a clearer picture of they compare. Before about 310-bits or so, SMT turned ON is about the same or better, but after 310-bits or so SMT turned off starts to do significantly better.
By the time we get to 360-bits it does 3X better, that is to say, SMT OFF runs in 1/3 the time as does YAFU SIQS with SMT turned on. It seems this is the peak, It starts to level off after that.