Even without NVLink C2C, on a GPU with 16XPCIe 5.0 lanes to host, you have 128GB/sec in theory and 100+ GB/sec in practice bidirectional bandwidth (half that in each direction), so still come out ahead with pipelining.
Of course prefix sums are often used within a series of other operators, so if these are already computed on GPU, you come out further ahead still.
Of course prefix sums are often used within a series of other operators, so if these are already computed on GPU, you come out further ahead still.