100 RPMs is the rough rule of thumb for a start, but I'm sure this wont apply to those bigger (over 500hp). I'd consider 100 RPMs per and inch of pitch the rule for a motor that tops out around 3000-3500 RPMs.
Unless you boat is very over weight it sound to me that you could swing and additional 2" of pitch for sure. You need to photo tach the boat to make sure that you are hitting your 3300 RPMs. At 22 X 20 seems wrong. In addition you boat must be 16 knots cruise at best.
used to have a 22x22 , changed it to a 22 x 20, i run at 2600 rpms. it is a heavy boat and as you said a little under propped ? my feeling is i don't care about the speed i would rather be safe with the overloading issue than sorry.