Sorry if i missed this, but how do you know? Or is this from previous performance of your boat?
If you are going by manufacturer quoted figures - i would assume that they don't always tell the 100% truth, maybe they had \ you had 2-3knts of tide when took that reading.
If it is a manufactures claim, do you have exactly the same prop? Is it in perfect condition and prefectly balanced? Just because it says 19x17 ect doesnt mean it hasn't been tuned and changed by a prop repair company. If the boat wasn't new to you, the previous owner may have wanted a quicker-to-plane time etc etc, or just tweaked the prop on the Training Bank and had to have it made smaller to be balanced.
Sort out the coolant issue - as that is a worry.
I had a coolant issue with a past car, it would loose water, and i thought the worst as it never leaked any on the drive! Answer was: when up to temp \ full coolant pressure, the seal on the water pump would weep - at rest or cold, no leak!
Rob