One goal of SP1 is to reduce the CPU-bound nature of FSX ....
Another goal of SP1 is to increase the multi-core nature of FSX and get benefit during rendering. How much of that we get in SP1 is still to be determined.
The performance increase will come in the form of less CPU overhead and there is no way to 'gauge' what that will mean because they can not define it in a 'number'. It is based on the system the update is applied
SLI may see benefits because that is what is currently holding it back, but with SP1 and a good reduction in CPU bound addressing, the system must be run at a minimum of 2400x1600 with 16xAA enabled or SLI will give no benefit in FSX, at all
there is no CPU available to the public that can handle 2 8800GTX in SLI anyway
In any case, anything that reduces the CPU load will be a relief to many
The biggest problem I see with SP1 is with all those who hacked the heck out of their textures and autogen files. That may have been a very, very bad mistake and could very well require a complete reinstall of the product. It will depend of the contents of the update. I for one tried all those hacks to see what all the hoop-la was about and as far as I am concerned, they were the worse thing I ever tested. I