105.9 Newark was never a full B, at least not in our post-1964 understanding (50 kW/150 m) of a full B. The original WHBI-FM from 1962 was authorized on what amounted to a contour-protection basis against several other stations, first-adjacent 106.1s in Philadelphia and Long Island and co-channel 105.9 in Hartford.
Those stations all became grandfathered against each other when the new rules took effect in 1964. WHBI-FM was able to move several times within Manhattan afterward - IIRC, it went from the Chanin Building to the Chrysler Building to Empire - but always with the requirement that it not increase interference to or from those other pre-1964 stations.
(The loss of the original WHBI-FM license and the "new" license that replaced it didn't affect anything technically; 105.9 was allowed to continue as though it were still the original grandfathered 1962 license.)
Not only is 105.9 limited today by those other pre-1964 signals, it's also wedged in by newer signals that have appeared since under current spacing rules, including 105.7 in Manahawkin NJ and 105.5 in Paterson NY. If memory serves, those stations have to protect 105.9 as though it's a full B1, but the pre-1964 protections actually limit 105.9 to rather less than full B1 status.
Bottom line: not only was 105.9 never a "full B" from New Jersey, unlike 94.7, it can't become a full B, or even a full B1, by moving from Manhattan to NJ.
As for WNYC's level of satisfaction with the 105.9 signal, that might be gauged by WNYC's plan to purchase a small noncomm signal (WDFH 90.3 Ossining) to increase the reach of the WQXR programming into Westchester. I suspect that if a better signal than WDFH were available, WNYC would go after it quickly. 105.9 isn't cutting it up there.