There are obviously very technical considerations beyond the scope of this forum but yes theoretically power consumption is related to the cube of fan speed - divide your RPM by 2 and you divide your kW by 2^3=8. Actual achievable energy savings would be a bit less but if you can get away with lower fan speeds by all means do so. However, retrofitting your dust collector at ~$1/operator-day savings might be an unacceptably long payback. And “get away with” is up for interpretation. Personally, I want the maximum flow rate possible in my dust collector for as much help moving the <0.5 micron dangerous dust as possible.
Lastly you may be disappointed with the sound savings. Here’s a chart showing the relationship between flow, static pressure, and sound:
It’s in metric so I’ll convert. 4 inches of static is about 1000 pascals. 0.5 m3/s is about 1000 cfm. So on this generic graph that’s about 98 db. Cut your flow rate in half to 500 cfm (~0.25 m3/s) and your sound pressure is still 95 db. Your mileage will vary but don’t expect amazing reductions in sound.