Cutter rig upgrade

[Brion Toss]

What material are the runners? How long are the tackles when they are set up -- that is, how far above the deck are the upper blocks?
I am with Brian on leading to the winch; 5:1 is an almost decorative amount of purchase compared to the loads it opposes.
As for your concerns about snapping the mast, it is highly unlikely, even with the current setup; your mast was designed for sloop loads, so it is fabulously strong for a cutter.
One other option I'd consider would be a Solent stay, which doesn't need runners at all, and which also allows for a hank-on drifter. So you can bracket both sides of the Genoa's range on the Solent.
Fair leads,
Brion Toss