Please note that if you attempt this, this is not officially supported by us here at Frontier. If this method functions, then it functions... and if it doesn't work... we won't be able to help too much - if at all. [happy][up]
The suggested SLI settings in the linked thread doesn't do anything, at least not for me. All it does is make use of both my 980 Ti but there is no performance increase.
I'm not sure that they've announced anything, but what I do know is that the game is very CPU dependent. I just upgraded from an i5 4460 to an i7 6700k and it is a huge difference.
I doubt that sli will help that much.
And also there will always be slow down in a full park, that's just the nature of this type of game.