1. Torque output on the new 2L is up over the outgoing 2.5L. Hopefully Subaru recognized that the increase in torque should be coupled with stronger drivetrain components. Of course, that was not the case in 2006, and if it's within reasonable tolerance, they won't spend the R&D.
2. In one of the initial reviews (don't recall the source), SOA informed the reviewer during a hard launch "not to worry" and that "the transmission could take it." That is comforting, to a degree.
3. From my own observation, it seems that the underbody shots of the 2015 WRX reveal a shorter bell-housing on the transmission, which may have necessitated the switch to cable-linkage. While this doesn't necessarily mean the transmission will be stronger, it points to the fact that Subaru may have intentionally reduced the input shaft length, which remedies a problem with the 5MT. Not a strong argument; just an observation.
I highly doubt the new 6MT is as strong as the 6MT in the STi. Subaru still refers to the STi 6MT as its "heavy duty transmission," suggesting that the value proposition for the STi may still be the transmission. If the new 6MT is even as strong as the outgoing 5MT, color me happy. I suppose it's up to us early-adopters to find out, perhaps the hard way