There are a few parts you can't see that are missing from the 83, mostly to do with the LPF and the MUX it switches through, afaik. Other than that it uses the same parts, including the same A grade of LT1790-2.5, which does make me wonder what accounts for the specified difference in performance in the non-AC ranges. More careful calibration and adjustment at the factory?
The Hi-Res mode on the 87 is sw rather than hw, as evidenced by the bug in the lower current ranges.
As long as the MCU is reprogrammable I guess you could just load an 87 fw, add the parts, and have an 87V.