>> The transfer won't be faster mainly because it's really I/O bound.
>> But it will use less CPU, only because you're making less syscalls.
>
> Have you actually measured this?

"""
vanilla over Gb/s:
real    0m9.035s
user    0m0.523s
sys     0m1.412s

block-sendfile over Gb/s:
real    0m9.683s
user    0m0.253s
sys     0m1.212s

full-sendfile over Gb/s:
real    0m9.014s
user    0m0.059s
sys     0m1.000s
"""

As you can see, the throughput doesn't vary (the difference in "real
time" is just part of the variance).
However, the CPU usage (user+sys) is less for block-sendfile than send
loop, and less for full-sendfile than block-sendfile.

"""
vanilla over loopback:
real    0m3.200s
user    0m0.541s
sys     0m0.702s

block-sendfile over loopback:
real    0m2.713s
user    0m0.248s
sys     0m0.197s

full-sendfile over loopback:
real    0m1.718s
user    0m0.055s
sys     0m0.082s
"""

Same thing for loopback, except that here, zero-copy makes a
difference on the throughput because we're not I/O bound, but really
CPU/memory bound (and here sendfile of the complete file really
outperforms block-sendfile).

I don't have access to a 10Gb/s network, but basic math hints that
sendfile could make a difference on the overall throughput.