Hi Jacko and welcome. I’m using four VMG1312-B10As in modem-only mode, router functions disabled, and I am using PPPoE over ADSL2. I run our own contributor Johnson’s PPP MTU 1508 (1500+8) firmware and it works great.
The reason that there’s no possibility of fragmentation here in my case is that the whole thing is inside a PPP and Ethernet tunnel. Intermediate devices don’t see IP packets - the PPP PDUs sent could contain anything, not necessarily IP at all. I transfer IPv6, IPv4 and PPP LCP PDUs too. As a real example of something other than IP, PPP LCP PDUs are used as the equivalent of an ICMP ping / echo request and one is sent every second or so by my ISP in order to test that the link is working properly.
I get a tiny efficiency increase because I can cram 8 bytes more IP payload into every packet which helps to overcome the bloat of all the headers. In theory there might be some device out there that insists on sending 1500 byte packets so it’s to your advantage if you can handle them, but this is not something based on a known real scenario.
Johnson also out some other juicy upgrade features in that firmware build for me too. Have a poke around in the various threads. There’s one concerning maintenance, new versions too.