Mitigation at parcel 27 cannot be a whole lot worse than what Wynn is going through in Everett for the casino site. At least someone probably has a map of what was buried in parcel 27!
There is also reasonable solid land throughout parcels 25, 26 a, b. Those are not all air rights. The Kneeland Street frontage is about 80% solid land, I believe.
One big cost factor not often discussed is the need to mitigate tunnel exhaust fumes from buildings on the site. Big exhaust stacks and careful HVAC design are needed to keep the buildings from sucking in all the pollution from the 93 tunnel. This was one of the cost issues that torpedoed the school proposal for parcel 25.
I agree, 25 and 26 are doable easily. The areas to the east and west of 27 are pipe dreams though, especially without doing some sort of foundation on 27.