In Summary, yes, it does. But adding too many washers will lock the gearbox and you will suffer a bit.
Also, as I discovered, cutting the softer side of a non linear spring and replcing it with a stiffer part of another spring will also increase FPS/MPS/Joules
but it is not very good idea as these springs can besh up over time and you will actually lose power.
Motor: Warhead Ronin 31K
Spring: Custom M160
Platform: E&L AKM
Cylinder Head: FPS Airsoft 2 O-rings
Gears: 100:300
0:00 - Intro
1:08 - Before Test
1:16 - Changing the Spring in the E&L AK
2:12 - After Test
2:18 - Gearbox Locked up
5:49 - Another Test
6:00 - Conclusion & Motor Connector Fix