Challenge mode normalization question

The explanation can get a bit technical, but basically no, you don't need a separate set of gear for challenge modes.

When your gear is scaled down, we keep your hit and expertise % the same, and if needed, subtract extra from your other ratings while keeping their proportions the same. It's also worth noting that we've added 1 level to every enemy in challenge mode dungeons, so that the bosses are 93 instead of 92.

These numbers aren't accurate, but just to illustrate how it'll work: If you have 15% hit, 10% crit, 10% haste, and 10% mastery, and your gear needs to be scaled down by 20% to reach the normalization threshold, normally you might expect your stats to go down to 12% hit, 8% crit, 8% haste and 8% mastery. Instead you'll end up at 15% hit, 7% crit, 7% haste, and 7% mastery. (I know that not all stats require the same ratings so this isn't precise, but hopefully you get the idea.)

Now, I did lie a little bit: If you're very precisely trying to hit some haste breakpoints for your spec, you may end up wanting to keep a separate set of challenge mode gear, but that's the sort of behavior we expect from people who already have Gold and are trying to improve their time by a second or two for realm/region leaderboard bragging rights. But the impact of a stat point or two here or there is going to be trivial compared to basic things like how your group handles patrols, how much or little crowd control you choose to use, and so forth. If you think back to something like the original Zul'Aman bear run, success or failure was never a matter of having 1% extra crit or not; it was more typically a product of whether you let that scout make it to the drums, or whether you got too reckless and wiped to one of the bosses.