For the 2nd test (after you got it up to 18 MB/s, which is higher than you'd be able to get if it were still in 100 Mbit mode), did you check if either side is CPU-bound? Rsync takes a decent amount of CPU, so that could be the bottleneck on gigabit networks.

You can also try adding the -W option, which will send the whole new file when a file's been changed, instead of the usual rsync diffs, which can be faster when bandwidth isn't the bottleneck.

Rsync adds a lot of overhead- The fastest way I've found to transfer a file is actually the venerable netcat.

On the receiving side, we open a socket, and direct it into a file-

nc -p 5678 -l > file.name

then, on the sending side, run

nc 192.168.47.1 5678 < file.name

That avoids the key exchange, the overhead of compression, the CPU to check for diffs, etc. Just pure copy.

I'd be curious the difference in speed you get.

If you like netcat, look at socat(1). It does everything nc(1) does and more.

Set up an rsync daemon so it doesn't have to go through SSH.