I don't think that a steel cable can handle the F16 loads, never mind the sheeve at the top.

On a F16 you can easily pull with 500 kg = 1100 lbs on the halyard ring and hook. I've broken two halyard shackles already and now sail with a beefed up Taipan 5.7 halyard shackle. This one is holding. There is no way any sheeve up the mast can hold that as it will be under twice this 500 kg load = 1000 kg = 2200 lbs.

Personally I have no troubles at all with my ring and hook system ones I tweaked it right. How you tie the halyard to the ring is indeed very important. There is a lengthy post about this in this F16 forum. I think it was covered about a year back. Look for it.

With regard to the 1:12 internal downhaul, get it. Best upgrade in the package

Wouter