for 10% improvement, and double the cost, why?
Better to take that extra money and buy a better card in the first place.
Nvidia claims up to 90% performance gain from SLI.
Back in the days when SLI configurations would only run at x8/x8 your statement was probably true. Technology has changed and the best motherboards can now run SLI at x16/x16 with both slots in 2.0 configuration.
So why not 100% gain you ask? Because one card has to act as the controller coordinating the output of both cards to the system.
Obviously, if your running a x8/x8 or a x16/x8 configuration your not going to see as much of a gain as x16/x16.
EDIT: Of course the offset to this is that, if your card can already handle everything you throw at it without slowing down why bother? Mine does and I'm still SLIing my machine just because I can.
