This is not going to be anti-LFR and you will see why is that with what I will suggest later on. However as it stands, I believe its easy accessibility defeats its design-purpose.
By definition
It is called "legendary" because you or with others need to do something legendary. Its counter-to-logic if you obtain that so easily.
By ilevel
It has the highest level in game; that means it should be the hardest part of your gear to acquire by logic itself; getting your "curve" in Mythic should be a standard requirement as it would be the ultimate reward of achievement or alternatively some type of mythic-kills requirement.
By design
This huge DPS boost implies that it is needed due to a wide range of extremely difficult, complex and diverse boss-encounters and mechanics or generally an activity that without it, it would be impossible to take part and perform.
Suggestions
a.Each difficulty; LFR, Normal, Heroic, Mythic should have their own versions of it and its ilevel with its procs should scale up accordingly; 5%, 10%, 15% and 25% with the appropriate ilevels.
b.Make it so that regardless of the difficulty there is some form of grind into it to accommodate for everyone; taking into account the latest raid results, it is pretty clear that most generally like to do to one difficulty setting, letting the 10-15% working to progress from one difficulty to the next. It should be something to "grind for" like a mount/transmog/achievement and thus be a complimentary motivating factor.
An example of 6.1 Legendary Ring.
An example of 6.2 Legendary Ring.
These are my initial thoughts.