Tough question when you limit it to only the plinth. The Scoutmaster also has a different motor and larger platter, so the whole package works together.
I did the Scout to Scoutmaster upgrade last year and am still very happy. I've upgraded the phono pre a few times since, and it's kept up all the way to the Tom Evans Groove.
The actual Scout to Scoutmaster (as basic as the plinth, motor, platter) upgrade gives you a good amount more detail and much lower background noise.
I also added the SDS and a Gingko Cloud 11 around the same time. The SDS was the most significant upgrade IMO and should be mandatory for anyone even thinking about upgrading their Scout. The Cloud 11 was good, but not as significant as any of the other upgrades I made around that time.