I'd say the Legendary drop rate is directly linked to the MasterWorks' one, so yeah probably. But given the mere 50% increase, GM1 is still more interesting to farm for legendaries.
I have to say I've done a few Freeplay this week-end and I've had a lot more legendaries than during the complete week of strongholds. Plus, the missions' variety compared to a repetition of Tyrant's mine was refreshing, even though there are a lot of potential bugs during Freeplay (including two that made me lose my loots, so now I'm just leaving the freeplay when I'm getting a few mw or one legendary to "save" it).