2 NAS servers both with 802.3ad bonded gigE nics based on the Realtek 8169 chip.
The highest MTU I could set was 7000 even though the D-Link DGS-1210-24 Rev. A switch can support up to 10k.
Below is just a single sample, but all tests stayed within 57x Mbits for MTU=1500 and 77xMbits for MTU=7000.
The important bits.
iperf was used for this testing.
[ 3] 0.0-10.0 sec 687 MBytes 576 Mbits/sec
[ 3] 0.0-10.0 sec 926 MBytes 777 Mbits/sec