I too would like to know how the Stick+'s doubled RAM improves performance. I just have subjective feel...
But I wish Roku had put in more channel storage flash in their 2017 players and TV's that don't have microSD expansion. I'm seeing the percentage "Loading" prompt more often (worst on the 256MB Stick, but also some on the Roku TV, and now even the 512MB Stick+ and Ultra once in awhile). Seems modern (SceneGraph/NDK?) apps are getting larger, and apps like YouTube, Sling, etc have a noticeable wait to redownload the app code even with fast Internet.
For that reason, I wish I hadn't purchased some of the 256MB Stick's and had gone higher-end.
I think minimum 1GB storage on the lower-end models, and 2GB or more on the higher end, would provide a more consistent user experience now and in the near future (and displace the need to bother with a microSD slot and user trial-and-error for a compatible card).
No Fire TV has ever had less than 8GB storage (along with 1-2GB RAM), though to be fair Roku OS is supposed to be more lightweight.
Shouldn't bring up the 32-64GB Apple TV or 16-500GB Shield TV (with unlimited adoptable storage) since they're in a different price class...
P.S. Would be nice to have a secret screen that lists the full app sizes.