Mantis Comfort Synthetic is about as comfortable as one can get next to gut. Even more, because it isn't so high powered, you can get away with lower tensions making it even more arm friendly. I tried it at 23kg or about 51 lbs in a PK Ki 315 and it played just fine. 24kg or 53lbs is perfect for me.
3X PK Ki5 315
Tour Bite Soft 17