Even with a P67 board that has the extra NF200 chipset, with triple-SLI it runs at x16/x8/x8. In some of the tests, they say it gets inconsistent results -- this is probably why.
An X58 board has more PCIe lanes, so has more bandwidth. If they did an actual comparison of triple-SLI between the two, X58 would win. We already know X58 wins when in dual-SLI (just barely) at large multi-monitor resolutions.**
**Source:
HardOCP