I've used this splice, even on old line. Not bad, so long as you loosen the rope
up first. Practice with some scraps first.
Though I have not pull-tested it, other have. Moreover, halyards are sized for stretch, not strength, so you are not going to be near the limit.
The other thing I have done, to save a couple bucks, is simply use the cover of the retired polyester line. Most often the part that was hidden inside the mast
is pristine. Use the old core
to pull the Amsteel through. Then just tuck the cover in at the crossover. Easy.