Most ZTRs have engines that are setup for 3,300 RPM or 3,600 RPM at high idle (full throttle). 3,300 RPM appears to be an attempt to extend engine life verses 3,600 RPM engines, not sure if this has been effective.
Mowing must be performed at full throttle.
You can determine the blade tip speed fairly easily by checking the spindle pulley RPM at full throttle, PTO on, with a photo tachometer and the blade length. Photo tachs are plentiful and reasonably priced. If you have one or buy one, remove the battery when not in use. A small piece of reflective tape is stuck onto the pulley which reflects so the photo tach can work. Belt/pulley cover must be removed to expose a spindle pulley. a R/H or L/H pulley can be used to check RPM as you are seated on the running machine. Reinstall the cover after checking the pulley RPM.
The formula for determining the circumference of a circle is C(circumference) equals 3.1417(Pie) times the diameter(blade length in inches). This gives you the blade tip travel for one revolution, multiply this by the RPM of the spindle pulley and you will have the blade tip speed in inches per minute. Divide this by 12 and you will then have the blade tip speed in feet per minute.
You had commented about swapping pulleys to increase blade tip speed. Manufacturers change deck pulley sizes between different deck sizes to maintain but not exceed the maximum legal blade tip speed. As has been mentioned, this not a recommended. The Big Dog Alpha and the Alpha MP do not share the same size decks and may have several different spindle pulley combinations to achieve max allowable blade tip speed.