I run a slash 6.0 on 11.5 to 16"
No but a new spring is often part of the upgrade.
The question is what are you trying to fix / improve?
Last I checked, what S&J and ATRS had avaliable weren't really that heavy.
I would get one if it was building a new lower, but other then quality of finish which would result in the buffer riding smoother, I would not expect much different results over stock.
I wouldn't bother with anything under 5.0oz personally but it all depends on how your machine is gassed.
You idealy want your brass casting to 3 - 4 oclock.