With cache, RAM and somewhat CPU binning, though market demand sways it too, it's not so much what speed grade it is, which is minimum guaranteed performance, but what the next speed grade was at time of production, which says if you might get it running faster or not. If the next one in this case was 3ns then with a bit of luck you might get close to that, if it was 3.2ns, then it won't go far at all, because all the chips that did that got pulled out of the supply and labelled 3.2. We saw it with PC-133. When PC-133 was new, the expensive stuff was 7ns, but some 8ns RAM would get there, because the timings fall between 7 and 8, however after a couple of years 7.5ns appeared which was really all the 8ns chips that didn't quite make 7, but 8ns after that wouldn't do 133, because all the better chips between 8 and 7 had been marked 7.5 instead.
Also you get an extra chance with top tier stuff that it was qualified at maximum temperature and the bottom of the voltage tolerance, meaning if you keep it cool and your volts aren't dragging arse, you might make it do the next speed grade. Then also you get a bit of a shallow inverted bathtub in the yields, early on the chips just make it, but then the fab is making really excellent stuff, then towards EOL the masters are getting tired and stuff starts declining again. So early parts, not much chance they go faster, mid production parts, wheeeeeeeeee, late production parts, sad panda.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.