The only time i have done this i got the best results by running mainstage on one computer and live on another and then hard wired the two together and used midi over lan to sync them. If you don't have two computers, but you do have a sound card with a pair of ins and outs that you don't need, try running cables from the unused outs to the ins you have assigned to live on the soundcard, then assign the in/outs to mainstage.
If your sound card has 8 in and 8 outs.
Live is using out
1/2 and in
1/2 3/4 5/6.
Disable in 7/8, and out 3/4 5/6 7/8 in the live preferences.
In mainstage select the same sound card.
In the input of the channel you are using select 7/8.
In the out put of MS master track select 3/4.
On your sound card cable together the out puts from 3/4 to the inputs of 1/2.
If i have thought this through properly you should get a signal from MS into live.
It will still have latency, but probably not has big as when you are using SF.
Although you might have to play with buffer settings.
Give it a shot any ways...