Verral maybe you don't understand how the group finder works. Very few goup leaders are taking time to look at logs. They need 1 or 2 players to round out a team and 9 apply. They pick the 1 or 2 they need and everyone else gets a message from Blizz saying "Your application was declined." Sometimes we pick the highest item levels, sometimes we pick the spec with brez or heroism or aoe stuns. I run with ret pallies regularly - great dps and good team mates - so I wouldn't reject you at all... but I might very well select someone else without ever getting to know how good you are.
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I tank normal mythics @880 when they include world quests for artifact power. I usually set the minimum I-lvl to 840 since we're not on a timer and that's what's recommended, but I do list them as "Mythic --- with 880 Tank" just to help them fill faster, and that tends to attract other geared players farming the same WQ.
If you want to be known for your performance rather than your item level, join a guild or at least make some friends so people will get to know how good a player you are. Strangers aren't mind readers and won't take time to look up your logs. Guild mates and friends will take you even when you aren't the best-geared person available, because they know you're fun to run with, that you'll interrupt and focus the right adds, etc.