I have a 109 disc priest twink. Some ilvl infos....
BG's scale to ilvl 780. if you get even a single point above 780, you will notice a stat increase.
BG's reward 780 gear than can upgrade to a blue ilvl 790.
Green quest rewards are 780 that can proc epic 800's.
Blue quest rewards are 793 than can proc epic 803's.
WOD mythic boa trinks are 805.
109 boe epics are ilvl 810.
101 boe epics go all the way up to 850.
I bought a boe for every slot useable by a priest and the 850 dalaran ring (250k gold)
850 waist (cost me 100k :<)
845 ring
835 head
835 hands
835 shoulder
810 trink (I have the 805 boa trink as well)
810 cloak
Some of those pieces have leech (I have 10% total leech) and the cloak has a socket.
Anyways, I haven't touched any quests yet (since they scale to 109, u can't use them to level) and I'm already 810. After questing I'll probably be 820 I guess. Maybe BIS would be 830 ish if you had all 850 boes.
edit; Here's the pros/cons of twinking at 109 verse 99;
Pros;
-You can spend your order resources on +165 art power missions from your class hall
-Faster BG queues
-You can actually queue random dungeons to receive +35 art power
-You can kill mobs in surmar and receive loot from them...not sure about the implications of this yet.
Cons;
-Can only get a max of 5%(?) increase in stats in instanced pvp whereas 99's can get up to 16%
-99's will always be more OP in world pve due to secondary scaling.