A few things to understand about the performance of SLI:
When people say performance may be worse. They may mean driver support is not be optimized for your specific use and this can lead to worse performance. Certain design software also won't utilize SLI at all and apparently neither will Valve games on the Source Engine. If you're playing games with this system you can expect the driver support will be acceptable for most mainstream titles as Nvidia is pretty good with staying up to date.
They could also be talking about "micro stutter." Micro stuttering is an interesting phenomenon as some people can't even see it, while others complain it makes games unplayable. Just do a quick google search in "discussions" for micro stuttering and you'll get the idea. Basically micro stuttering is a side effect of the way SLI renders frames. That is alternating frame rendering where card A renders frame 1, card B renders frame 2, card A renders frame 3 and so on. So if your monitor is plugged into card A when B renders a frame it needs to traverse the SLI bridge to be displayed on screen.
However, because of how alternating frame rendering works, in a well optimized program Nvidia can claim 100% performance increase. Which is true because a single card is only doing %50 of the work it would if it was solo.
Nvidia recommends you use the same exactly the same cards as the clock rate between aftermarket OEM's on the Shader, GPU, and VRAM can differ. However as a side note I and many others online have had success using the same GPU with different VRAM in SLI. I was able to use a 3gb 580GTX with a 1.5 580GTX (both from EVGA) without any problems. The memory is mirrored so it gave me a total of 1.5GB Vram, it just a waste of the extra VRAM.