With the current engine weighing about 435-lbs and the Mercury 150 ProXS weighing 455-lbs, and both rated for the same horsepower, I think the choice of which engine to purchase for re-power so as to have the least effect on transom weight is clear: the Mercury 150 ProXS
You can move weight out of the stern of the boat by moving all batteries to the console.
The boat speed can be predicted using the Crouch Speed Predicting Method. I have created
a calculator to implement that method. The method requires knowing power, weight, and hull coefficient.
The effect of horsepower on top boat speed for this boat can be estimated with the calculator.
This discussion lacks useful data. In this discussion there is no mention of the boat's top speed with the 150-HP engine. I will assume the boat can hit 35-MPH.
In this this discussion there is no mention of the total boat weight. I will calculate the boat's weight by assuming a hull coefficient of 190: 4,420-lbs.
Now the horsepower needed to increase boat speed by 10-MPH to 45-MPH for a boat that weights 4,420-lbs with a hull coefficient of 190, can be predicted. However, I will add 70-lbs to the weight to account for the larger engine: the required horsepower will be 252-HP.
To increase speed by 5-MPH will require 200-HP.