I was considering something similar as well, but seeing that the differences are not that big, I thought it was a good idea to at least keep the decoding behaviour the same.
The thing is, the gap between -6 and -7 is currently that small too, at least, with my dataset. Currently, changing from -6 to -7 increases the compression by 0.083 percentage point at the cost of dropping the speed by 0.46x With this proposal, changing from -6 to -7 increases compression by 0.065 percentage point at the cost of dropping the speed by 0.65x
----- Setting ----- , compr , encsp, decsp
-8 --no-exhaustive-model-search -A tukey(0.5);partial_tukey(2);punchout_tukey(3), 56.511, 64.0 , 379.9
-7 --no-exhaustive-model-search -A tukey(0.5);partial_tukey(2);punchout_tukey(3), 56.647, 74.5 , 403.0
-8 , 56.690, 60.1 , 379.0
-6 -A tukey(0.5) -A partial_tukey(2) , 56.712, 115.2, 399.9
-7 , 56.833, 84.2 , 402.6
-6 , 56.916, 182.6, 402.3
But of course, I'd like to hear what your results are! I was just seeing the very small difference between -5 and -6 and the gain between -7 and -8 and thought: maybe this could be spread out more evenly.