Do you have any hard statistics on this type's general population prevalence?
Nope.
In all likelihood, nothing even remotely close to such statistics exist.
But one can do a back-of-the-envelope estimate, based on statistics that do exist, and a few assumptions.
INTJs are somewhere around 1%-3% of the population, depending on which source you look at.
6s are ~20%-25% of the INTJ population, based on the only statistics that I believe exist (and almost all those would likely be 6w5s).
And, lastly, sx's are believed to be the least common of the three instinctual variants, with sx/so being less common than sx/sp.
I figure the ratio in society to be something like 50%-60% sp, 30%-40% so, and ~10% sx.
That would make sx/so's, at most, be ~5% of the population.
More realistically, they're probably about half, or less, the population of sx/sp's.
So something like 2-3% for sx/so's wouldn't be a horrible assumption, and likely nothing higher than 5%.
Assuming sx/so's aren't any less common amongst INTJs than other MBTI types (which may not be a good assumption), this would yield, at a low, something like .005% (1% INTJs x 20% INTJs as 6w5s x 2.5% INTJ 6w5s as sx/so's = 1/20,000), and, at a high, something like .0375% (3% INTJs x 25% INTJ as 6w5s x 5% INTJ 6w5s as sx/so's = 1/2667) of the population being INTJ 6w5 sx/so's.
The actual number probably lies within that range, and no higher than 1/1333 (shifting the last assumption all the way to 10%).
Take the average of the high and low ends of the range, and you get somewhere around 1/12000, or .0083% of the population.