Using the more common 4d6, drop low method, the odds are 1.62% (vs. 0.46%) -- pretty much exactly 3.5 times as likely. If you get six stats and arrange as you please, the odds of an 18 prime stat go up to 9.34%
That sounds like a lot: 934 out of every 10,000 characters -- almost one in ten. But, let's assume that the "big four" classes and their sub-types are evenly distributed. Maybe not strictly true, but close enough for rounding errors. That means 243 characters out of every 10,000 (2.43%). If each group played AD&D for 15 years and rolled 25 characters per year (which is high for my experience, but whatever), that's 375 characters over the span of the edition. Each table should have seen roughly 9 characters with exceptional strength. Probably not absurd to have the rules, but not worth a lot of space/effort, either. Best to have dropped it.
Yeah. I like statistics.