The basic reduction nozzle rules are; the larger the nozzle exit diameter, the greater the volume of water you can move. This is great for a freestyle bottom end and hole shots but the problem is that sometimes you do not have the proper exit velocity for top speed. The smaller the nozzle exit diameter, the greater the velocity of the water exiting, the better the top speed. The problem is that sometimes there is not enough volume of water to get the ski to top speed. It takes a blend of the two that gets both hole shot and top speed. If you have two different exit sizes or angles to compare, then one is more than likely better than the other and it takes testing to find out what works best with your setup. All the nozzles have a tapered exit angle and it is NOT recommended to bore them straight because it loses pump efficiency so the angle should be maintained if boring your nozzle. I'm sure each reduction nozzle that Yamaha develops is specifically engineered for their skis by the engine power to weight ratio, pump size, impeller type & pitch, hull design, top speed, drag etc. Most of their jet-ski’s with smaller displacement engines that weigh less use the steeper 25.5-degree reduction nozzle angles with varying nozzle exit tip angles. While the bigger ski’s that weigh more with the larger engine displacements use the 21-degree reduction nozzle angle with different nozzle exit angles and sizes. Some of their nozzles even use a bowl shape design with the nozzle exit angled upwards. Your hull can travel at radically different angles at different speeds depending on your ride plate, venturi angle and even how you're positioned on the ski. The steeper nozzle exit angles of 3.5-degrees or more will lighten the nose of the craft and may add top speed. While the shallower nozzle exit angles of 2.5-degrees or less will deliver improved performance in rough water conditions by producing more nose pressure to drive the hull through the bumps which is perfect for closed coarse and offshore competition race boats. Also a ski traveling at an angle of 2-degrees does not bring in the same amount of water into the intake tunnel as a ski traveling at 5-degrees at real high speeds. The faster the ski travels the greater its tendency to run flatter and use a smaller venturi. If you continue to try to run at 5-degrees, then you're going to run into the pump over stuffing problem and a bigger diameter venturi may be just what you need to process that extra volume of water coming into the intake. Mag pumps use larger diameter hubs which increases the pump pressure slightly since there's is less area typically the exit nozzle diameters don't need to be increase as much. All these things need to be taken into consideration when using other reduction nozzles types with different angles and exit diameters.
Here's a chart to see all the angles and dimensions of the different type reduction nozzles.
