You've been able to do straight c1s since forever :P. Yes, there is evidence c1s improve xp/hr, although 1-30 is rarely exactly the best, but it's a convenient standard. Actually since batch 2 of dungeoneering.. Before batch 2 you could start at c1, next dungeon would be c2, next c3 etc etc.. - Back then doing c6s was faster than doing c1s. I guess I jsut have never done pres runs since batch 2