I get that by moving my footstraps out I will be able to hold the rail down better when its lifting at speed. Are there any other advantages?

Moving Footstraps Back - Advantages?

I like to get on the rails more for loading up the fin (laterally) rather than keeping the rail down. Upwind improves & control (at top speed) in out-of-control conditions also improves.

Every now & then I might swap boards with mates (chop hopping & blasting), and I struggle with inboard straps, always end up with just my toes in the straps & heels on the rails...maybe I'm a closet formula-rider?

For speed-sailing I'd definitely be moving the straps outwards.

At the risk of stating the obvious have you tried using a smaller fin on the days when this happens?

full outside and full at the back : top speed :::

I moved the straps as far out and back as they could go on my freerace board and could not believe the difference.
You have more control over the board probably because of the leverage, and I think you have a better feel what the fin is doing eg when it is about to spin out.
Also goes faster