Hey, Jam,
Yes, turn the IS Off -- not sure why you're getting different results with the 1Dx, but I have not had good results with any camera with IS On while on a tripod.
I am still testing and trying to figure out my 5Ds. Using IS off tripod, shooting at shutter speeds that gave me sharp photos with a 5D3, I am getting serious motion blur. I think this camera will benefit from rock solid tripods and technique.
Hey, I started with a pair of D30s too, then switched to the 1D as soon as it came out, then 1D2 then 1D4. Got a 5D2 for personal use then a 5D3 for work, now a 5Ds for work. You'll get used to the 5D series quickly enough. I expected to hate the 5D2 based on hating the 40D, but actually liked that little camera. I still have the 1D4 bodies for sports, but can't imagine using them every day anymore.