So the outcome of this is that I had a resync at 10:46 this morning which removed the DS interleaving and enabled G.INP - on an ECI cab!
I need to read up on G.INP to see exactly what it does, but it appears to have taken DS sync within about 3 Mb/s of max attainable, while keeping ES similar to when interleaving was applied.
This is a good result as far as I am concerned.