Oh, there's the confusion. Experienced Marksman is a PvP set which just happens to be colored purple in the merit vendor and only sells at level 50. PvP recipes drop at lower levels.
What I think is commonly called purple are the very rare sets: Absolute Amazement, Apocalypse, Armageddon, Coersive Persuassion, Fortunata Hypnosis, Gravitational Anchor, Hecatomb, Ragnorok, Soulbound Allegience, and Unbreakable Constraint.
Those are only available at 50 and can only be slotted by a character at 50. Attuned or not, and really should have attunement disabled for them.
And is a Panacea at level 50 attuned the same as a level 50 Panacea, yes. The Heal/Absorb version is 42.4% for both. Though a 50+5 is 53%
PvPs are odd in that they can be gotten at lower levels, and if attuned will level up with you, but at level 50, they're worth swapping out and putting in a +5 version.