No it is not guaranteed to have better performance.... you will find some games for example that already get 80 fps with a single card as it makes no sense to develop an SLI profile for that game... what you do get is a huge performance boost in games where it does matter. Gains over 100% have been realized (Far Cry 4 - 3840 x 2160 Ultta)
http://us.hardware.info/reviews/6292/6/nvidia-geforce-gtx-980-ti-sli--3-way-sli--4-way-sli-review-ultra-hd-in-ultra-quality-benchmarks-far-cry-4
This is how much faster two 970s are at 2560 x 1600 in the following games
Thief 67.82%
Far Cry 3 67.80%
Tomb Raider 67.71%
Battlefield 3 66.07%
Bioshock Infinite 63.52%
Crysis 3 61.57%
Splinter Cell: Blacklist 61.47%
Battlefield 4 56.69%
Batman: Arkham Origins 56.11%
Watch Dogs 49.04%
Grid 2 48.48%
Assassins Creed 43.06%
Crysis 42.31%
Metro LL 40.23%
By comparison I don't much care that ....
-WoW: Mists of Pandaria is only 38.40% faster w/ two 970s than a 980 as they cost me the same
Diablo III: Reaper of Souls is only 20.66% faster w/ two 970s than a 980 as they cost me the same
Wolfenstein: New Order is actually 9.06% slower than a 980 because I am already getting 56 fps w/ SLI disabled and it has a 60 fps cap !
-Id much rather play Tomb Raider at 58 fps w/ twin 970s than the 980s 35 fps ...
-Id much rather play Far Cry 3 at 69 fps than the 980's 41 fps ...
-Id much rather play Crysis 3 at 43.3 fps than the 980s 27 fps ..,
.... and play all those others 40-67% faster than a 980 even if it means playing Wolfenstein at 4 fps below the game engine's 60 fps cap.
It's not that games don't exist for which SLI brings nothing to the table, it's just that:
a) the ones that don't, performance is already at a point where it doesn't matter
b) the game is obscure and not very popular.
All AAA games will certainly have an SLI profile and one that produces significant scaling