I still have my TanTan x38 frameset (Scott Foil RC clone) sitting in a box that I disassembled over a year ago.
Size 56 weighed a whopping 1148g and was stiff enough. The problem was dynamically the back end was pretty sloppy and easily disrupted over rough pavement. But I was kind of speedily rushing through bike builds two summer ago. Perhaps I'll rebuild and give it another shot next season.
Having used the same wheels on x38 as my Giant Propel, the Propel is quite stiff too. But in a way that's just more agreeable dynamically. This is my fear with the TanTan x68 (SL8). Past trends will almost always predict future trends. In the case, marginal disappointment.