would GS and setup combo really be worth the points in a Ambush/BS build where (if things go as planned) they should be dead without swinging many (if any at all) hits at you?

I have no clue about ghostly strike but since is this is your first post I have to ask how you can hate cold noodles when soba is so absolutely delicious!!!

The deal with the subtlety tree isn't that setup is so wonderful, it's that you get so many great skills on the way to setup. Preperation, Imp Sap, Imp Rupture, Initiative. Setup is just the icing on the cake.

Now personally, since Imp Ambush and Imp BS both increase crit chance by a lot, I feel Seal Fate has better synergy than Setup.

I think you answered your own question here. If you are planning on killing with very few swings and very quickly, then I wouldn't expect your opponent to be swinging very much at you...which Setup is 100% dependent on your opponent swinging at you so you can dodge.