Should mention that Purple (Very Rare) and PvP set bonuses go down all the way to level 1 with you even if the power isn't available when you exemplar.
This means attuning (or buying attuned) Purples is a waste of time as the level you can slot them doesn't change, the set bonuses already exemplar down to level 1 without being attuned, and if you attune them, you can't booster them.
PvP IOs is a different matter due to their level requirement of 10-50, so since you can start slotting it at an early level, it may be more of an advantage for you to buy attuned version if you desire slotting partial or whole sets for the set bonus, and if you're min-maxing, replace the attuned with normal version and booster it when you get to 50.
ATO set bonuses go down to level 7, if I recall, and I don't know if the power needs to be available or not. Not sure on event enhancements, haha.