It isn't about the impact this gear will have. It depends on how available it is (you're making a lot of assumptions in your post about this, by the way), but it is likely this will have little to no effect on progression.
I want to bring some 'rough' statistics into this.
According to WarcraftRealms there are 145796 level 85's.
241 US Servers, is roughly around 600 people per realm. Cool, that's awesome.
Now from my knowledge (Please correct me if I'm wrong), an auction lasts 3 days, 10 auctions roughly per cycle.
From this, there will be 100 auctions per month. A tier will last 4-5 months, that's 400-500 auctions.
I want to say we can assume 1-2 items per cycle will be heroic tier items, that's only 40-50 of them being available through that period. That's not..alot. For your specific class, 3-4 items. Barely an upgrade.
Edit: Just saw MMO SS of it, could possible be 4 items. 80-100 per tier - 6-8 items over the tier possibly available for your class.. I am hoping some will be transmog gear though.
As I said, I just thought of this up since personally it doesn't seem a huge impact. I'm pro this, mostly because there's no gold sink currently, I'm an AH freak and I do some heroic tier (4/8 before boredom break).
What's your opinion on this?
Also it's in a PvP zone, going to be fun trying to lock this down. I am envisioning some amazing guild vs guild fights for BMAH control.
Also Lore, please tell Gary to calm down. I used to love legendary, I watched this show for the first time in about half a year... I'm quite disappointed...