Well, there, that’s certainly better. Thanks for pointing out the duplex mismatch. My bad for not considering the automatic fallback to half-duplex mode which still uses CSMA/CD.
Setting the speed to 100 and enabling autonegotiation did the trick. Now it is only unfortunate that GbE will not work, but that is hardly a showstopper for me. 
Again, many thanks!
osmc@osmc:~$ sudo ethtool -s eth0 speed 100 duplex full autoneg on
osmc@osmc:~$ ethtool eth0
Settings for eth0:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric
Supports auto-negotiation: Yes
Advertised link modes: 100baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Link partner advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Link partner advertised pause frame use: Symmetric Receive-only
Link partner advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: MII
PHYAD: 0
Transceiver: external
Auto-negotiation: on
Cannot get wake-on-lan settings: Operation not permitted
Link detected: yes
osmc@osmc:~$ ./testnfs
Testing transfer speed of udp on file 192.168.179.43:/raid/random100m.bin
/mnt/test mounted @ rsize (8K), requested rsize (8K)
104857600 bytes (105 MB) copied, 9.57727 s, 10.9 MB/s
/mnt/test mounted @ rsize (16K), requested rsize (16K)
104857600 bytes (105 MB) copied, 8.84943 s, 11.8 MB/s
/mnt/test mounted @ rsize (32K), requested rsize (32K)
104857600 bytes (105 MB) copied, 8.86104 s, 11.8 MB/s
osmc@osmc:~$ ./testnfs tcp
Testing transfer speed of tcp on file 192.168.179.43:/raid/random100m.bin
/mnt/test mounted @ rsize (8K), requested rsize (8K)
104857600 bytes (105 MB) copied, 9.78113 s, 10.7 MB/s
/mnt/test mounted @ rsize (16K), requested rsize (16K)
104857600 bytes (105 MB) copied, 9.13921 s, 11.5 MB/s
/mnt/test mounted @ rsize (32K), requested rsize (32K)
104857600 bytes (105 MB) copied, 9.01669 s, 11.6 MB/s