SSG gives about 3% avoidance on top of the yummy Sta it grants. This is assuming you maintain the 565 defense. Since 540 defense is the problem you are solving at the moment, you are going to loose the bonus avoidance. However you are freeing up a gear slot then you are gaining whatever you are putting in the gear slot.
What it boils down to is what do you want to gear for? This is a question I ask myself on a boss to boss basis. Some bosses need more threat (Hodir hard mode) while some bosses need as much EH as possible (Heroic ToC content comes to mind...). I carry around an extra bag or so of tank gear to meet the needs for differant fights.
I'll try to be nicer if you try to be smarter.