Does the higher 1500 MTU make much of a difference in speed?
1464 bytes to 1472 bytes is only a 0.55% increase.
It might be possible to use the HG612 in router mode with 1500 MTU with a few short commands.
I don't recommend the HG612 in router mode on FTTC unless you have a very low sync speed.
It doesn't have the guts to do higher speeds when doing PPP/routing at the same time.
On my old 80/20 line the HG612 (in router mode) wouldn't get anywhere near 80Mb, struggled to maintain 40Mb, and dropped down to 20Mb when uploading.
Great modem, not designed to be used as a router though.