Short answer is NO, you won't get more speed.
Your motor will be turning a little closer to it designed/engineered "wide open throttle range" which will cause less build-up (carbon) and your hole shot/time to plane (may) improve somewhat, but you are only making 31 mph because that's all that combo is likely capable of.
If you want more speed you are going to need another motor.
Pitch is roughly translated into "how many inches of forward motion you will get for each rotation of the prop" - but that's in it's purest sense.
If your motor is turning at 4600 rpms and (to keep it simple) you have 2:1 gears, then the prop is turning at 2300 rpms.
2300 X your 19" prop, without slippage or resistance to forward motion would give you 43,700 inches (forward) per minute, or 3641 feet per minute or 41.3 mph - since you are only making 31 mph, you are "slipping" about 25% (actually more than that because I believe your gears are closer to 1.8:1) - a little high which leads me to suspect that a 75 horse motor on your particular boat is leaving it a little "under powered" - 15-17% "slippage" is more typical.
Now, if you went to a 17 pitch, your rpms (at the crank) would increase to 4800. That would mean "about" 2400 rpms at the prop.
So 2400 X 17" - 40,800" per minute - 3400 feet per minute - 38.6 mph - so less then the "theoretical" 19 pitch, but should slip less (just don't know to what degree) - if your slip rate dropped to 20% (7.2 mph loss) your "top speed" would be 38.6 - 7.2 OR the SAME 31 mph you are making with your 19 pitch.
Looking at the specs for your boat it's rated for 120 horses max.
Typically you would fit a motor that is "minimally" 80% of the max to get the boat to operate "as designed" - so 100 horses would better suited to your boat, and a 115 would actually make it run the way it sounds like you want it to.
That 75 horse "Force", which while it would have kept the cost down when new. But with a boat tipping the scales at 1600 pounds (without passengers, gas, equipment, stuff etc), you ARE under powered - in light of that, 31 mph sounds actually "pretty good" for the way you are rigged....