The prediction is that precocious puberty should correlate to a lower IB. There doesn't seem to be any real study into IQ scores for children experiencing precocious puberty. I only seem to be able to find a study by John Money claiming girls show a higher "Verbal IQ" (Which, if you know anything about John Money, you know is worth less than nothing), another in 1984 with a sample size of 103 girls (52 precocious/51 normal) agreeing with John, and a third one in 2016 with a sample of 30 (15 precocious, 15 normal) finding the precocious have an IQ of 94.
Long story short: Nobody tested their IQs, and what little they did test doesn't actually map to IB.
The IB model doesn't account for hormones at all. I don't know that myelination pace is at all affected by them. Affecting hormone levels downstream with drugs should have no impact at all on the actual process of maturation. That said, as the brain myelinates, critical developmental windows might be missed. The current generation of children would be the guinea pigs to find out just what part of the experience of Puberty at its natural age is essential for development.
You lightly de-myelinate their thalamus, somehow- maybe their whole brain, then pharmacologically suppress re-myelination. The second part is feasible. The first part is still science fiction; the best anyone can manage is total, uncontrolled de-myelination, which will kill you, if you're lucky.