the weight of a 15 versus a 7.5 is in reality about 10 lbs.... no big deal.... or if u use an 8 they are the same..... weight is a non issue..... and the added speed will be about 4 mph... also neglegible.... the difference lies in the fact the boat will do top speed with a full load versus a single person setting just right...
as for the engine trim level.... you want the anti ventilation plate AKA cavitation plate.. even with the bottom of the hull, and propeller paralelle to the bottom of the boat.... and if steering torque is excessive adjust up or down 1 notch or hole to cancel it out..... if the engine pulls right go up if it pulls left go down...
what you may find is that you get excessive steering torque and raising the engine may cancel it out but the propeller begins to ventilate and cause the engine tto rev excessivly, if that occures the propeller needs to have some cup added to hold the water against the blades so the air bubbles comming of the hull and gear case dont get around the blades the cupping is easilly done for a neglible fee at most propeller shops and honestly on a small aluminum prop it takes about 10 min..... a skilled person with a brass hammer and trailer ball can do it at the boat ramp....
