I'll try to go in order.
1) Yes... if you trim up, and lift the nose... you get more of the hull out of the water, and in theory, you will have less drag, and make more speed. The problem is... nothing in the world is free... so to lift the hull takes power. Also, as you lift the hull, all the weight is being supported by a smaller, and smaller area. So, eventually it starts to create drag. When you trim up... there is always a "Sweat spot" for performance. One more thing... you may find the perfect trim for smooth water, but as soon as you hit a ripple, you may start to bounce.
2) Wedge: If you trim the hull using the nozzle... then you are also loosing power because you are redirecting the water. SO... what we do is to wedge the pump. This is a little ring that is cut at an angle to hold the pump nozzle are a differt direction. SO... if you find that a little up trim gets you more speed... then you put in a wedge with a few degrees of "Up" in it. That way your steering/trim nozzle is in line with the pump nozzle, and you get back that power.
3) wear rings: If your ring isn't in bad shape... you probably aren't loosing much. Normally, when they are lightly warn... you don't loose top speed, but you do loose hole-shot, you get more cavitation from a stop, and your top RPM is higher. It's always advised to replace the ring if you are putting in a new impeller. In a bigger, heavier ski... it's a different story.
4) Grates: OK... with a jet ski... you can't have handling and speed... so you have to chose. Almost everything to improve handling will scrub speed. I don't really recommend removing the grate, but if you are looking for every last drop of top speed... removing it helps... but you will slide out in a corner, the pump will come unloaded, and it won't hook up in rough water.
This comes to the flip side of all of this... once you tune for peak performance on smooth water... you will loose ALOT of speed once it gets rough. (We will talk more on this in a minute)
5) OK.. the rep has his head in his ass. He is not running 67 MPH with a stock engine, in an X4 hull. The only thing he said that was right, is if you put on a steeper pitched impeller... you need to open your pump to get the RPM back. Basically, what it comes down to... your engine can only make so much pressure in the pump. So... if you go a degree or two higher on the impeller... your exit pressure will still be the same, but you will loose RPM because you are running out of HP. This can go to the point where you fall off the "Peak" HP RPM. Opening the pump nozzle will allow the pump to unload, and you will bring the RPM back up to peak, and even though your pressure/velocity is the same... you will move more volume, and in turn, you MAY go faster because of a bigger push. (I hope that makes sense)
Note: Sorry this got long... but I am just trying to give good info. Now... on my personal ski, I run a skat trak Swirl that is one degree higher than recommended, and my pump is opened 1mm. BUT, I run a top loader grate, and I have LARGE trim tabs that are trimmed down 3 degrees. (pushes the nose into the water) With this set up... I can grab the throttle, and the hull will basically jump out of the water when I hit the throttle. Also, even in rough water, my hull stays planted, and my pump stays loaded... so in turn, I run fast on rough water. Unless you weigh 90 Lbs... you will have a problem with porous'ing at speed being trimmed up. When that happens, your pump will unload, and you loose top speed... also, you will have to let off the throttle to get control back.
Now... in theory, the top loader, and the tabs will scrub speed... but I'm getting a solid 55~57 mph even in rough water.
As a final note... you asked why can the bigger GSX go faster. Well, that has to do with aspect ratio, and without a physics lecture... it's hard to explain, but this is why gliders, and sail boats work so well with minimal power. The simple answer is that when your length is extended, but you stay narrow, then the parasitic drag is reduced. But like everything else... this is a give and take since you reduce your turning radius. (The X4 hull will out corner the GSX)
