DPS wise here's an example. Let's take a very geared guild that could comfortably beat Patchwerk's enrage at 60, killing him in six minutes when the berserk is 7. 4.2 million hp, 360 seconds, 11666 raid dps.
Looking at old screenshots of mine, early in Wrath people were doing around 2,500/4,000 DPS on Patchwerk, or 60k raid dps in this case.
Factoring out tanks for the moment, with 26 dps in your 40-man patchwerk raid and 16 dps in your 25-man raid your dps are doing about 10 times as much damage at 80 as 60.
Kel'Thuzad in his last phase at 80 has 5.84 million Hp. Kel'Thuzad in his last phase at 60 had 1.75 million Hp.
To be tuned the same as 60 Kel"Thuzad 80 should have 17.5 million Hp in his last phase, and that's not even taking into account that due to the tank being mind controlled you had to full stop dps with chains coming.
That's like doing the boss with a 66% health reduction.