Part of the answer will depend on the design and materials of the pendants. You can have enclosed steel pendants, or glass pendants that are translucent or fairly opaque. I have parchment pendants that give mainly downlighting as well as some ambient glow. Some pendants may give off a lot of light and others may not. What specific style pendant do you have in mind, and are you looking at downlighting or do you expect the light to project from the sides as well?

I agree with you...an island that is almost 8' long would benefit from more than just two fixtures. Two other things to consider: 1) how high above the island the lighting will be (higher will provide somewhat less light), and 2) make the fixtures as high wattage as possible and use a dimmer switch if you need to dial it down.