[JPonLKN]

Well, I guess first of all, they are designed to run around 15 MPH, and they are designed to run up to a point where the governor on the transaxle takes over and pushes the throttle closed a little to a point where it maintains that speed. Entirely normal and what it is supposed to do. I am guessing you found the technical manual...there is a section in there that discusses how to go about setting that. To begin with, download a GPS speedometer on your phone if you haven't already, turn it on, and run it while driving the cart. You should floor the cart and let it accelerate to top speed. If its getting over 15 and the governor on the transaxle isn't pulling the throttle back, you need to adjust the governor. For testing purposes and for short durations...you can't hurt the motor, as the ignitor has a rev limiter that kills ignition/spark...you can't over rev it. I wouldn't run it there for long term, but for testing purposes, it won't hurt the motor if you happen to get up to that speed. Before we go further...you should do that to make sure there is a problem. My mom is an old lady, and she doesn't like running her car at highway speed for fear of blowing up the motor. We need to take "Feel" out of it.
Now, it is possible that your driven clutch isn't shifting...
since you are running and driving it, I'll assume the drive clutch is closing and engaging the clutch properly, but the driven clutch operates based on "realized load". It should start with the belt flush to the outside perimeter, and as you accelerate, the clutch spreads apart and allows the belt to ride further down, shifting the drive train. It is called a CVT, or Constantly Variable Transmission. As one clutch opens, the other closes, and vice versa. If you are starting with the belt at the top edge of the driven clutch and it doesn't open, you will rev up, have a ton of torque, but no speed. All the ones I have experienced started where they should, opened up on speed, but wouldn't upshift or close as I would approach a hill, basically keeping it in high gear when it should down shift. I took one apart and the button screws were all broken. Maybe Trig, Cart, or CP will chime in on your situation, but I have never seen one "stuck" and not open up. I've driven several and they only really seem like they are screaming as you approach 20 mph. 21 is about max before the ignition rev limiter in the ignitor kicks in and shuts down spark. I didn't have a tach on them, so I don't know what RPM that happens at. I would guess between 3500 and 4000.

If you have a workhorse, your driven clutch will be a 28 degree unit and not a 36 degree one. It provides more torque. The 36 degree units are provided on TXTs. As the diameters are the same, overall top speed for Workhorses and TXTs are the same. The difference is the rate of change of the belt diameter for a given "experienced load" on the drive clutch. The 1200 in the MPT name refers to the weight capacity for the machine. It is designed to haul quite a bit. If 15 isn't fast enough for you, upping to 20in tires will get you a few more MPH for the same RPM. Going to 22" or 23" tires should be just fine, but I haven't done it. I think Cartmaster has, but I think he is running the 295 head on his 350. I'll let others chime in on tire size and usability as mine are all 18" so far. My next task on my 1200 is to mount my lift and 23" tires, so I will be able to answer after that, but haven't done it yet.