I don't think BD-RE speeds will be increasing faster than 2X anytime soon. I think it's a technology wall they've run into. Maybe it has to do with dye reflectivity. This is why DVD+RW is still at 8X and DVD-RW is at 6X.
I have two BD-RE DL 50GB discs and they take a very long time to burn. I use them for data backup only.