Can’t speak for the boosted, but the standard Rainier plays quite well for a 6 series monometal when you put a beveled bearing in it. I like mine quite a bit! Good stability and power in general, not just for a performance organic. You can get through a 30 second combo and have it coming back to your hand with no trouble at all.
The statement about comfort is also true. The Rainier generally has a bit of hang time to it, gently encouraging you to chill out during play. I say generally because this can change based on the setup you use (bearing & SEs).
I have heard that the only difference some have had between standard & boosted versions of the Rainier is that the boosted one feels more aggressive in play. So you may want to go with the standard if comfort is your highest priority. That said, my theory is that the boosted will also have a bit more power based on the increased density of 7075. OneDrop claims that the boosted has more power than the standard one, which has good power already.
Edit: this’ll help: