The average ilevel thing is pretty useless, since you can game it by holding on to stuff you don't wear, such as bfd quest junk. I'm also not sure how it interacts with heirlooms.
Nonetheless I don't think gearscore or ilevel mean anything to us. Our gearing pretty much goes in a predictable progression that is easy enough to spot with an inspect or even a glance at their HP.
- Not in dungeon BiS
- Dungeon BiS plus full enchants
- Missing 1-2 heirlooms, BoEs, LFH
- BiS and AGM
You rarely see anyone gear up in a different order, except that AGM comes before full heirlooms on servers other than AP. The difference in return-on-effort-investment between each of the steps is almost mathematically logarithmic, with #1 taking 8 hours for 65% power, #2 taking a couple days for 85%, #3 taking a week or two* for 95%, and #4 being a month long multi-person endeavor for your last 5%.
Anyway, my point is that you don't really need an addon or a number so much as a glance at their HP and shoulder/weapon graphics. You can reasonably assume a 1400hp pally with an Arcanite Reaper probably has his quest BiS bracers, too.
The whole concept of gearscore is for 85s where everyone looks the same visually in their tier/season, and each slot has a zillion intermediate upgrades that linearly advance their power. F2Ps pretty much fill all but four slots in their first 48 hours /played.
*but seriously, fuck LFH