Reducing to x8 will not make a difference. Just like Snipergod87 said, even using a super powerful card like an ARI 5970, you would barely notice the difference. With my 6870 at 16x and at 8x, I noticed no difference at all. Same FPS, same marks in 3DMark06, same score in Unigine Heaven Benchmark.
Bottom line, no difference between 8x and 16x.