Did you mean to say there WONT be a difference? I am looking for more torque to pull the rake and tiller without revving the snot out of the engine all day...
Here's some food for thought.
The industry standard for OPE engines is 3600 RPM. In other words, Honda, Kawasaki, Yamaha, Onan, Wisconsin, Briggs and Stratton, Robin, Tecumseh, Kohler and others ..... all set the governors to maintain 3600 RPM at wide open throttle (WOT). In addition, the camshaft, carbureation and ignition timing are all designed to produce the maximum engine torque just below this RPM.
Implements that spin (tillers, mower decks, snow blowers etc) are designed to perform at their peak efficiency when the engine is running at WOT. Case and Ingersoll tractors use a hydraulic pump to power many implements. Once again, if you want the max from these attachments, then you have to feed them enough hydraulic oil to make them spin up to their design speed and that only happens when the engine is running at WOT.
The hydraulic system will only produce X amount of HP and X amount of torque regardless of the size and hp rating of the engine powering the pump. Many people feel that the max hp any engine can make the Case hydraulic system produce is 12 hp.
Quixote2 told you in his post that if you wanted more hydraulic hp, then you would have to redesign the whole system and he's right. Now if it's engine RPM that bothers you, then find and install an engine that produces 16 hp at 1800 RPM. That way, you can set it run only at speeds up to 1800 (half-throttle) but with it powering a 20 GPM pump, you will still get sufficient flow to run attachments.
Will you have more torque at the wheels? Nope. But the engine will be quieter. The problem with that scenario is that you would have to change the pulley sizes for belt-driven equipment if you wanted to use them a tractor with an 1800 RPM engine.
And you thought it would be easy..... yes?