I bought an SDS over a year ago used on A'gon for about $750. I feel it's been a very good purchase.
I don't have a Scout, but a HW19 MK III with a SAMA (stand alone motor assembly). So, essentially my set up is similar to the Scout.
The motor on the HW19 appears to run very smoothly and this is even more true with the SAMA. When you turn on your TT with it plugged into the SDS your TT receives 115v for start up and then within a few seconds it drops to 72v. If you have your hand on the motor you can feel the outside of the motor housing suddenly go from what seems like perfectly quiet to what is much quieter still and vibrationless.
Plus, with a good strobe (KAB Strobe) you can use the SDS to fine tune your TT to perfectly 33.333 RPM. And, with all the new 45 RPM LPs coming out you have instant settings for 45 RPM as well.
I think the SDS was more important previously on the old HW19's with the motor affixed to the TT base because it made the motor quieter. With the Scout motor or any separate motor it probably is less important.
But that said. I feel mine makes a noticeable improvement still and I'm glad I purchased it.
Other changes to your Scout might be more noticeable, I can't say, but I can say I like my SDS.