It's all the same bosses from lvl 60, just more hp and hit harder for lvl 80 raids. Of course they will clear it quickly. If they want a real challenge, gdisband, split up and join casual raiding guilds. Then they can see how hard end game content can be with normal/casual players, with only 3-4 hour raids only 3 times a week.

If they want an even bigger challenge. Move to Australia and try raiding with Oceanic latency.