23 spline to 18 spline

HP has nothing to do with wear on a gear or spline. Torque and shock do, though.

The shear torque withstanding on a 23 spline is somewhere in the neighborhood of 750ft lbs.

This changes with shock (how you clutch) and how sticky the tires are/ how much wheel spin it has.

The real question is; what are you doing with the car? The gear ratio is the appeal behind an 18 spline input shaft transmission. The shaft itself is not that much stronger than a 23 spline.