Well, graphics cards are routinely run on 8x speed when SLI is in use - there will likely be a small performance penalty, but I wouldn't think its huge - else SLI wouldn't be worth it.
Its a bit apples and oranges but with a different (and I believe newer) graphics card - The AMD Radion HD 7970, Anandtech showed very little practical difference between x4, x8 and x16 in PCIe 3.0 (Yes, its a newer version of PCIe as well. Finding these benchmarks is hard)- since PCI-e 2.0 is half the speed look at the x4 graph for x8, and x8 for x16 . While yours is nvidia, Its a lot older and would need less bandwidth.
I'd also suggest doing a benchmark with some tool that compares it against other similar systems and getting a definitive answer. I'm guessing it should work acceptably under most workloads. Considering the scenario, it does seem worth doing.