The S60 and S85 show the same 380 HP but the S85 is about 400 pounds heavier and is .5 quicker 0-60. Based on this the actual HP on the S85 must be higher. It's also about 8% less efficient on range per battery size. EV Trip Planner shows weight has little affect on range. So if range isn't affected much by weight then the main difference must be the higher HP of the S85. If this is the case the range of the P85D should improve if it runs on the 221 HP motor only.
It's not that simple, and you're connecting the wrong pieces. The S60 and S85 have identical motors and inverters - only the battery pack is different. No one has been able to explain to my satisfaction the difference in the two car's freeway efficiency ratings - I actually started a thread on that a while back.
Although we don't know all the details, from Tesla's comments we believe that the front motor is geared much taller, which makes it potentially more efficient at higher speeds - GM says they gain 2-4% efficiency on the Volt by dropping the main motor rpm to about half using the second drive motor.
I think your final conclusion is correct, but the logic chain you used to reach it isn't not.

Walter