I play with two other Legends regularly. Though we all have the L79 Bladez, I have the L81 G20 driver and 3W, and it is only the 3W that provides me any real advantage, and that is only for certain holes with certain conditions. Just today, I had a friend outdrive me with his L48 R1 on a few holes. He also was using the L59 G20 irons until he finally gave in and began learning the L79 Bladez.
The upper level Clevelands, IMO, are a HUGE difference, but still not enough to give a sub-average player any advantage over a stellar player who tiered earlier than the "average." The L79 Bladez provide a bit of advantage, but only if you are hitting the ding or close to it. I truly do not see any big advantage the equipment available to a L80-L89 Legend would provide for the average/sub-average player that would leave the players able to tier earlier than that at a disadvantage. Against equal skill level players? Yes, but not by a huge amount. IMO, it is not until you get to L90 that the equipment makes a huge difference (L90 R11 irons, L94 R11s driver, L95 R11s 3W, L98 R1). There are some pieces in the 80s that can provide benefit, but only if you are hitting the ding well, 'cause the penalties for not hitting it can be severe.
What level does WGT feel we should tier up at? I think that is a great question. I would guess that... before L75 is early, L76-L85 is average, and above that would be late. All IMO of course.