Yes and no, the problem is that there are two unknowns, the veridical per-packet-overhead and the gross rate of the link. The latter is limited nby the sync, but since most ISPs seem to use a per-subscriber traffic shaper, the sync often is not the relevant gross rate.
The problem is now that underestimating one parameter can be compensated by overestimating the other. To test whether a selected combination of rate and overhead works well, one can perform bufferbloat tests at different packet sizes, by e.g. using MSS clamping on the router....
This ended up being VDSL2 (as shown by the AT&T modem's status page), and I'm getting great results with Ethernet / 26 bytes link layer adaptation at 90% of my non-QoS speed. Thank you all 