Ok, so I'm a bit late on the draw for this reply, but I finally got around to checking them out on my system.
First off, using Youtube for any sort of comparison isn't the greatest source. I generally only listen to FLACs or CDs directly. Secondly, on my system (Foobar2000 -> Unison USB -> Gungnir MB -> Burson Soloist 3X w/ supercharger), both tracks sound super smooth to me. With lesser Delta-Sigma DACs (especially ESS chipsets) and THX amps, I would bet that there might be some sibilance. I'm sensitive to it and in the past I have had my ears abused by "sh" and "szzzz" sounds on many tracks on other systems and even my floorstander speakers with cheap DACs and amps. But in my current desktop headphone system, both of those tracks that you linked sound great with my Timeless.
What this tells me is that it's most definitely NOT the Timeless that's causing the sibilance. But it also tells me that if you guys are hearing it, then the Timeless are NOT covering it up, either. So, if you want a true transparent system, then my opinion is that the Timeless will give you that, and will not add any spike or sibilance in this frequency range. If you are hearing sibilance, and you want to get rid of it, then either upgrade your system, or get a better source, or find another IEM or headphone that compensates for it by covering it up.