If there even really perks for being sixth tier other than the bigger avatar (which is only ten more pixels on each side) and - while i'm not certain - a bigger profile picture? If not, then I don't see what would be the point in changing it personally. To me, after fifth tier is basically someone who is willing to donate more than just having perks, so adding an extra tier would be pointless unless we split (if there is even more than one) perks of teir six. But supporters don't need any more perks than they already have, personally. On any level.
I can see what one is getting at in people being more willing to donate if the milestone is $50, but with so little perks would the tier badge really be what one would want to aim for? I suppose it might be something someone really wants enough to donate so much money, but I don't think there is much of a big likelihood of that happening.
I feel some of this wasn't worded the way I wanted to, but my opinion is there, at least.