I've done a lot of research on this lately. Assuming you don't have any pre-existing thyroid issues, up to 500 mcg would probably be fine. Once you go past that, there seems to be an increase in TSH... an increase in TSH is not a huge deal but it is certainly not optimal, and some researchers even consider the increase as 'subclinical hypothyroidism'. Yes you read that right, there seems to be a fine line between iodine intake, and TOO much can actually be a negative.
A smaller thyroid volume is better than a larger thyroid volume, if I recall correctly. Iodine intake is weird in this regard. More iodine intake in the short-term seems to decrease thyroid volume, while longer-term studies (over months and years) show that those with a lower/more moderate iodine intake actually have smaller thyroid volumes than those with an excessive iodine intake.
Additionally, kelp / bladderwrack is interesting, I recall reading some studies suggesting that it may be good as an anti-estrogenic (reducing 17beta-estradiol levels). I believe this needs more human studies though.