Performance wise you shouldn't see much, if any, performance loss in an X8 PCIe gen 3 slot.
SLI requires 2 slots with at least x8 and x8 connectivity so in theory, you should be able to do so using slots 2 and 5, 1 and 4 or 2 and 4. You should even be able to do so with other combinations provided both slots used have at least 8 PCIe lanes but you would need single slot cards for most other variations.
Now I say in theory because there are other factors that could prevent SLI in different combinations than the ones recommended but I suspect it should work fine.
For single card use, any slot should work so long as it doesn't share lanes with other devices/controllers.