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Apparently the different level of Garrosh heirlooms level at different speeds, so at 100 "flex" version will be as good as Mythic version. At 100 they'll all be as good as Hc dungeon stuff.
Higher level heirlooms just give you a bit of a boost when levelling - though the difference at say 98 must be minimal.
I do preface all of this with "apparently" mind, as several people say "as it says in blue notes", but I can't find it.