Nikon Nordic (and by implication, also Nikon Europe as they supply the actual cameras) aim to sell D5 in its basic XQD configuration. A modification to CF *only* is possible at the appointed Nikon repair facilities. However, Nikon recommends against that as the full potential of the camera cannot be realised.
CF will have around 1/4 of the buffer capacity, 1/3 of the writing speed, and data transfer to the PC/Mac will take many times longer compared to XQD. Nikon considered modification to get CF storage slots a "degradation" of the D5 (their words, actually).