There is no truth in the 10% statement.
It's actually very vague statement because there are a lot of factors involved. Poliquin was just making a point. Because if we are talking about how much strength steroids can add, are we talking about one cycle, one year of use, 20 years? Are we talking about someone who could be assumed to be pretty much maxed out naturally and then goes on steroids, or a you guy who still has some natty gains to make?
I have observed a few cases. One guy, top IPF lifter, many years experience. A year into juicing with moderate doses, under a gram a week, about 10% gain in maxes, same bodyweight as when natty except leaner. In technical lifts where equipment helps a lot like sumo deads the gains were almost nothing whereas raw conventional went up almost 20%.
BB is right, the assumption that the more technical the lift is the less sterois help seems correct. Raw muscle strength can increase a lot but if the lift is more about technique and mastering equipment you might not get much. So it's hard to put exact percentages on it, depends on the lift, raw or equipped, bodyweight constraints etc.
If you look at powerlifters on IG you can see some insane things done by tested or even clean athletes. There are tested young girls pissing all over bodybuilders who have been juicing for decades.
For example, look at this girl's videos:
https://www.instagram.com/p/CGtXh_dgi18/?igshid=p5sngbyatzus