Well I think I'll give up on the D7800 and return it this evening. Tried quite a few things with Netgear Support (not too impressed with them now tbh!) including rolling back the firmware, nothing worked to give me good sync rates. My upload was actually some 50% lower than it is on the SBG3300-N.
Think I'll try a VMG8924-B10A, I assume it has the same Broadcom chipset as the SBG3300-N and will work with DSLstats?