40mm could work, but it's too close for the margin, you only have 5mm clearance on the 40mm tires, you need to take into consideration the tires could attract mud/sand/dirt, and those will be rubbing the carbon chainstay!!!
not a good idea for carbon.
I'm still holding out my recommendation for the RD301D, due to many hiccups along the way and lately found a serious design defect on the downtube storage cover. See my posts on the Long Teng thread.