Yes they are not that far off. The “actual” distribution appears to be in the ~30% stabilizing, 70% destabilizing range. That certainly doesn’t make stabilizing mutations rare.
Also, the vast majority (~84% of mutations) have only weak effects on stability, and so are effectively nearly neutral with respect to protein stability.
Another good paper on this is:
Faure G, Koonin EV. Universal distribution of mutational effects on protein stability, uncoupling of protein robustness from sequence evolution and distinct evolutionary modes of prokaryotic and eukaryotic proteins. Phys Biol. 2015 Apr 30;12(3):035001. DOI: 10.1088/1478-3975/12/3/035001
In the entire set of analyzed proteins, there are very few strongly stabilizing mutations (ΔΔG < -4 kcal/mol) (Figure 1). The effects of the great majority of the mutations (84%) lied between −3 and 6 kcal/mol, and 47% were between 0 and 3 kcal/mol. Given that the free energy of protein folding is distributed roughly between 5 and 15 kcal/mole [41, 42], these findings indicate that most of the mutations are near neutral (have a negligible effect on protein stability) or slightly destabilizing. As already pointed out, the distribution is skewed toward destabilizing mutations but strongly destabilizing mutations (ΔΔG > 6 kcal/mol) are rare. In T. gammatolerans , the fraction of stabilizing mutations, with effects between -3 and 0 kcal/mol, is significantly lower than in other analyzed organisms, and conversely, the fraction of strongly destabilizing mutations is significantly greater (Welch two-sample test). The distributions of the effects of single nucleotide substitutions show similar trends (compare Figures 1a and 1b).
CONCLUSIONS
In this work, we took advantage of the growing collection of protein structures from diverse organisms to examine, on the whole genome scale, the connections between biophysical characteristics of proteins and their evolution. We find that the distribution of the mutational effects on protein stability is a universal characteristic of cellular life forms, with significant deviation detected only in a hyperthermophilic archaeon. The results of this analysis imply that a protein’s robustness to mutation, at least on the whole protein level, is, to a large extent, to uncoupled from its abundance and sequence evolution. Such uncoupling appears to be particularly pronounced in the compact, apparently highly optimized proteins of prokaryotes. The less compact, more fragile eukaryotic proteins show a notably stronger connection between the relative protein core size, taken as a measure of mutational robustness, and sequence evolution rate, particularly at the protein surface. The general conclusion from this analysis is that the minimum required or optimal mutational robustness was reached at early stages of protein fold evolution whereas much of subsequent variation was neutral.
My bold.