Would the setup in the title image break when you upgrade to electric inserters?

Yes, the setup would break.

Would it be possible to allow normal inserters to do the fuel leeching too in the furnace to furnace configuration, or make it an option in the mod settings? I usually don't use burner inserters all that much but would love to use the fuel leeching for more compact steel smelting.

This feature is unlikely to happen with the mod in its current development form.

I will be approaching Klonan to expand the native (C++) inserter leeching capabilities.

Okay, I didn't know that burners work very differently from normal inserters.

They don't...

But when I approached Klonan about it he is extremely hesitant to utilise the logic on all inserters as it is a probable performance drain.
And quite a significant one.

so furnace to furnace is not an option?

Not via electric furnaces inserters no.