I asked a jetski guru the 12 vane is better for bottom end pop but will shed a little top end. For top end a bigger impeller pitch is the way to go if it can turn it.
I've been VERY involved in race gas, stroker, MSD, Skat pump, etc...standup jetskis for many years. IMO, the guy you talked to is mostly right. The only thing I'd disagree about is the top end loss. With the right prop, EVERYTHING is better with a mag pump. There are several variations. Vane count, size, and setback/non setback. By far the most common/desireable pump is the 12 vane setback. Setback pumps move the stator and prop rearward 1 inch, but require a setback shaft. This isn't much of an issue with jet Jon's since you're installing the pump assembly/cavity and motor mounts anywhere you want them. Between mid shafts and driveshaft lengths you can make almost any configuration work. Almost all yamahas use OEM 144mm pumps, with the exception of a few 760 sit downs and all of the larger motors. Interestingly, until you begin to make HUGE HP, nearly all 701/760 based boats use a 140mm mag pump in place of the stock 144. The stator vanes on a mag pump are stainless and can be be made thinner and straighten the water more gently, allowing for more nozzle exit velocity. There are a few "*******" 8 vane pumps out there and a fair number of 14 vane pumps, but most are 6 or 12. Either is so much better than stock that it is almost impossible to describe the difference. Think of John Force on his huge wrinkle race slicks compared to him running the same race car on cheap street tires. The difference is that great (especially as you increase power). The nice thing is that the trend in skis is HUGE(1500ish cc twins) motors that actually need bigger pumps. That means that there are far more 140's to be had on EBay for better prices. Even the 6 vane is VASTLY better than stock. With the 140, it requires the uses of a 140mm wear ring also. Skat bores the stock 144 wear ring and installs the smaller dia SS ring. The stator is installed the same way. SS stator installed in a bored stock stator housing. A used 140 will almost always come with the wear ring since it has to be used with that pump. They're also subject to corrosion issues, but the stainless ring breaks the aluminum, rather than the other way around as usual. The good thing is that Skat will install the wear ring or stator in another bored OE housing for about $100. If you happen to buy one new, the best thing to do is buy a chipped up stator and swollen wear ring and send it in to Skat to have your pump built. It'll save you several hundred dollars. I'm on the home stretch on a Tracker 15/42(?) jet Jon conversion (ill post pics when done) that's using a Skat setback 12 vane 140. Luckily I have 5 or 6 props to play with, but so far my experience is that you can run a much taller prop on a flat bottom Jon boat than you can on a ski. The Jon boat is a skipping stone almost the instant you touch the throttle so it's much easier to push. I've normally run a 13/19-21 on most of my skis (65 mph) but I'm starting out with at a 15/22 on this boat. The motor is just a stock 760, but will be running MSD total loss ignition, a bit more compression, a dried out stock pipe, and the mag pump. It should make a bit over 100 HP, so it's nothing crazy and can run on 50/50 AV gas/pump gas. With the right prop and nozzle diameter(stock is 81mm-most people bore the exit nozzle to 83-85mm) cavitation is a thing of the past and you'll be able to drive right through whitewater with no slip or spin. I'll see if i can get a few pics of the mag pump to post so the difference can be seen. Sorry for the dissertation, but a good mag pump setup is really almost too good to be true....except that it is. :lol: Hope that helps.
P.S. Regarding the all SS aftermarket wear ring, I'd personally buy a used OEM on EBay for next to nothing. They're a dime a dozen and usually really bad corrosion is associated with saltwater boats. I'd also personally stick with Skat props. Solas props are Chinese and use inferior stainless. That said, their mag pump copy isn't bad, but the base of knowledge for props, nozzles, etc. is far smaller.