Yeah, I seen others tune to under the claimed torque limit too. Good luck with that. The recommendation stands. Seen enough tunes under the max torque limits lead to new clutches. Manufacturers leave margins on the table for a reason. 15-20 Nm isn't much margin. That is a Margin of Safety of .08. I never was able to build anything with that low of a margin. It takes no account for process and part to part variation. You're assuming every part is a perfect part. In the engineering world, that is very far from conservative. A safety margin of one is acceptable. 1.5 or more is conservative.