Highest clock is the best for AMD64 regardless of the htt and multi used. But in your case, you can reach 2.7ghz with both setups. I think using higher htt would be better but you can test both configurations and see which one produce better results.

Im using Abit KN8 Ultra with E6 too. Currently using 337x9 with /12 dividers. From what i can see, the tightest timing would win. The mem bandwidth will benefit from it rather high clocks with loose timings.

Well, most mobos for nf4 will lock the pci clocks on default.