A motor with a lower RPM would definitely reduce the scooter's top speed in direct proportion to the amount of reduction in new motor's RPM.
Although, judging from when I first got my E300S with stock gearing (15 MPH), as I changed sprockets, it didn't seems like the top speed decreased in direct proportion to the "gearing", perhaps since as I decreased the load, the motor was able to "spin out" more (at a higher maximum RPM). For example, if with the 15 MPH sprockets, the top RPM of the motor was 2400, what then should happen at 2400 RPMs with the "8 MPH" sprockets installed? Should that be the new top speed or will the motor "see" a much lighter load and just spin past 2400 RPMs? I guess a good way to answer that question is to test the RPM of the motor shaft when there is nothing attached to it (or maybe just the front sprocket) but no chain (thus no load).
Most electric scooter motors are rated with no-load RPM specifications so they will not go above their rated speed when a lower gear ratio is used with them.
I currently am running 9 and 89 tooth sprockets on my E300S but need more reduction for offroading such as riding across grass. Instead of resorting to custom sprockets which are expensive, I was thinking or replacing the motor with a similar wattage motor, but one that spins slower. For example, using "round" numbers, let's suppose the stock E300S motor spins at 2400 RPM max and I replace it with a different 250 watt motor that spins at 1800 RPMs max. Also assume my E300S currently goes about 8 MPH on level pavement as a top speed. Since the new top RPM is 1800 and 1800 is 3/4s of 2400, does that mean my new top speed will be 3/4 of 8 MPH which is 6 MPH? If so, that is effectively like "gearing" it down better for offroading. At this point, it may be more cost effective to replace the motor rather than dork around with custom sprockets so I am asking about this motor swap idea.