Ditto on the motor being the same. My guess is that Tesla's using the same rear motor and gearing as the RWD/AWD, and the top speed and acceleration are limited by lower system voltage from including fewer parallel strings of 2170s. Tesla reduced costs on the MR by using fewer cells, using one(?) less charger board, and maybe downsizing some of the other power electronics.
Based on the difference in EPA range, Tesla can build 13 MRs for every 10 LRs given the same amount of cells, so they can balance Fremont's higher 3 production rates with GF1's lower pack production rates. More 3s rolling off the line means they can increase supplier parts volume and try to minimize part cost ahead of the launch of the SR. They can also maximize the number of people in the US who can get the full federal tax credit.
When GF1 can ramp cell output and the SR comes out, Tesla will have lower costs for the majority of the chassis/suspension/etc thanks to incrementally more production from the MR + LR, and if they really build the pack for 220 miles of range, they'll likely use shorter gearing to keep 0-60 acceleration at 5.6s and reduce the top speed, which is limited anyway by the smaller SR pack. Along the same lines I wouldn't be surprised to see a middle interior trim as well, although with enough volume Tesla may be able to go straight to the base interior.