While I'm sure this has been discussed before - but was there ever any
thought of running the London Overground trains along the SLL into
Victoria as well as into Clapham Junction? ie - one every 15 mins out of
Victoria, along the SLL to Highbury and then one every 15 mins from
Clapham Junction to Dalston or some such.

I suspect it must have been thought about, as it would provide some
decent options to different places out of Victoria, but presumably they
couldn't run 8 trains an hour down the SLL (plus 16 or however many
through the ELL core) and I guess anything less than 4 trains an hour out
of any of the stations is no-go these days.