After 2+ years of effing around with my own TT bike position, I can tell you from personal experience that it can be a HUGE factor in rider comfort and your ability to sustain an effective TT bike position. My seat is angled down a couple of degrees. If it were purely horizontal it would literally feel like there was a RR tie up my ass on the TT bike when I'm on the aero bars. Every individual is different. The fact that the UCI even has that rule just underscores what idiots they are. The fact that they decide to arbitrarily enforce it (after not doing so) on the day of the TTT in the biggest race of the season is just beyond the pale. Most of pro cycling's problem is that the organization that runs it are a bunch of pompous self-absorbed jackwagons.