It varies with speed. The problem with any belt driven accessory is that it generally (Unless it had some sort of variable drive mechanism) requires a fixed amount of torque but the RPM varies. So let's say at 1200 rpm it's 3hp, that's about 13+ ftlbs of torque. So now you increase it to your 3150 limit on your 2305, now it's the same torque but at more speed so it would take 2.6 times 3hp or 7.8hp. The problem is they usually calibrate belt driven equipment to work at idle speed so at higher engine speeds there is usually a ton of waste. Same is true on cars. If they calibrated them to get only the power they needed at higher speed they'd suck at lower speeds/idle. You can tweak this by changing the size of the pulley on an accessory to make it take less torque (bigger pulley) or more torque (smaller pulley) as long as you don't go so small you exceed the maximum RPM of the accessory.
The one exception generally to this is the Alternator since with speed the field strength (What causes the resistance to turning) can be reduced at higher speeds if desired.
