First general motherhood statement: As an in memory GPU database wherever possible we try to minimize the back and forth over the PCIe bus as we would like data to live on the GPU as much as possible (shout out to great blog here https://devblogs.nvidia.com/parallelforall/goai-open-gpu-accelerated-data-analytics/ talking about GPU Open Analytics Initiative (GOAI) which is leveraging this even further in the wider community)
With all that said we obviously transfer lots of data over the PCIe bus when we are initially loading the appropriate pieces of the DB to the GPU, and when we need to swap data off in the case of needing to bring new data to the GPU. So we try to have as fast a connection to the GPU is possible. I have not tested recently but I recall using a machine with 8x GPU connection and the transfer speed was basically halved from around 8 to 4 gig per second actual.
So the speed of the PCIe is crucial for a good users experience especially when loading large new datasets to GPU but maybe not as large as your initial thoughts on a steady state DB.
Did you mean a Titan Xp? i would recommend a Titan Xp, As far as Titan X to 1080Ti, unless you really need the extra 1GB of vram on a titan X I would recommend the 1080Ti over an older Titan X.