Using leveling gear when synced to that level

[MistakeNot]

If I'm leveling a class in the 60's and do MSQ roulette with the Ala Mhigan Earrings equipped, do I still get the 30% exp bonus even though I'm not actually level 50 or below and just synced to that level?

Yes, you will get that Exp bonus when synced down to level 50 or below.
Of course, you don't get much exp in the first place from the mobs in the MSQ dungeons, so you won't have much benefit from the Exp bonus there, but you will get the bonus.