One very interesting fact that I measured. With 0,8 mm mono line I had 2,6 X more energy of the spear, on 5 m, than with 1,7 mm line! Now I use 1 mm line. It holds 60 kg.
I think 60kg is too weak. If you shoot a big amberjack/grouper you will break the mono for sure...
1mm could be good for bass and giltheads during wintertime, but for other bigger fish I think it's too risky...
The speed and the power of the shaft is very important, but the most important is to take the fish back to home.