Overall, I like the idea, especially of reconfiguring the wasteland that is now the Canal St. area for future residential, etc. Up until now there has been a lot of wasted potential there.
Also opening up the location for the expansion of the daylighted canal is a good thing.
I'm assuming that Maitland St. will still connect to Portland, to provide an additional exit from the neighborhood (and the possibility for people using it to skirt around traffic?).
All in all I don't see any huge issues for traffic flow as rush hour traffic tends to flow from the right lane of Alderney to Portland anyhow.
At least they aren't installing roundabouts there...