Per my post above, 31.75, 31.7, and 31.8 are all exactly the same thing. And they are all really 31.75.
My Deda bars all measure 31.7 dead, yet my other bars from Cinelli & 3T, as well as the Enves in the shop I work for measure 31.8. Enve even advise against using Deda product with theirs for this specific reason.
Regarding ELITE X2 : 31.8 Road
Q – Will X2 work with 31.7 bars?
A – Yes, 31.8 and 31.7 both refer to 1.250 inch diameter. 1.250 inch converts to 31.75mm. Some people round up to 31.8, some round down to 31.7.
I suppose it's possible that ultimobici knows better than Thomson, but ...
As long as we're on a roll
The fact that 1 1/4" happens to be 31.75mm is irrelevant and not an indication of it just being a rounding issue. The predominant units of measurement in cycling are metric and have been for many a year.
Ever hear of 28.6, 31.7, or 34.9 diameter frame tubes? 1 1/8, 1 1/4, 1 3/8 inches, baby. How about 1 inch, 1 1/8 inch, 1 1/4 inch, 1 3/8 inch, 1 1/2 inches top and/or bottom steerer diameters? Oh, one last thing, how about 31.75, a.k.a, 31.7, a.k.a 31.8, a.k.a. 1 1/4 inch diameter stems?