It's definitely gonna make traffic worse at many hours of the day (although from 2pm-6pm, I anticipate it actually being better for those coming into JP on Centre, since that rotary is just dominated by all the traffic coming outbound on the parkway during afternoon rush hour), but I think it's the only way to go, and I say that as a frequent car driver who will likely be facing a couple extra minutes or more on every trip into the city.
I think any further lane reduction, though, would be a disaster, tho I agree those are some wide pavement areas. I do wonder if Parkman Drive requires two left turn lanes for the Arborway northbound? I know people use it a lot in the morning to try to bypass the Riverway, but two lanes seems potentially unnecessary. Probably needed, though.
My actual complaint here is there is a lot wasted pavement on bike lanes. Bikers are nimble and dont require whimsical extra lanes and X intersections. There are a couple places where two lanes diverge on the same piece of ground. Compress them, and dont pave anything for bike or car that's not absolutely necessary.