The quality and functionality of a frame is not so much its shape as the materials used and the workmanship, and there's a difference of day and night between the SL8 and the X68. If many consider (as I've read in this forum) these two frames remotely comparable, it's only because they use the bike for amateur purposes (like me).
Sorry, but I also need to comment this post...
We will all NEVER know what exact performance you can get out of SL8 and X68. It can simply be due to personal oppinion / feeling (which is not relevant) OR because we do not know X68 numbers OR we are simply shit riders that can't even use full potential of bikes.
I would also like to hear your arguments comparing X68 to SL8 as day and night. In what ? how do you back this comparisson up with some non-personal facts?
If we blindfold you, and put you on X68 with S-works stickers...are you sure you would reckognize which bike is it? =)
So, there are many of us that can absolutelly agree that X68 frame (700 USD) and SL8 (4-5K€) is more than comparable with ride feel.