I've finally started to play around with Challenge Modes, here's some notes regarding gear:
- Gear will scale down to 463 but not up, so if you have any piece below 463 item level, it stays at that level. But if you have epic or upgraded gear, it will be reduced.
- Sha Touched Gems are deactivated, Tier piece bonuses are deactivated.
- Pvp stats do not get scaled down, not that they do anything for you.
- Your hit and expertise are not scaled down, bosses will be level 93, but most of the trash in the instance is only level 91, so you might want to consider reforging to not miss on level 91 mobs and put the rest into dps stats.
- While pieces with sockets have their stats scaled down an extra ~9%, the gems you place are NOT scaled down. Higher level pieces with multiple sockets will give you an advantage when scaled down compared to pieces without sockets.
- Along the same lines, enchants and profession bonuses do not get scaled down.
- Off topic, but mass res is disabled.