It is possible for any boat to get waterlogged, if, for example, it had been submerged, or, if it had been stored out in the weather without a cover and the drain plug left in place, for a long period of time. Either of these situations would allow water to wick into any exposed wood beneath decks, etc. Bottom paint will decrease boat speed, but it would have to be especially rough, multiple, uneven layers to see a dramatic performance decrease. Normally, if a boat has bottom paint, it has been stored in water, and not on a trailer. If it has multiple layers of bottom paint, then you can be pretty sure it has been stored in water for more than one year. Have you checked below decks to be sure your bilge pump is working and you are not carrying water? Pull drain plug and tilt front of trailer real high and see if boat drains a lot of water. Check all of your below deck compartments, fishwells, etc for leaky hoses and drains, and after you're confident water is not collecting, then remove some screws from fishfinder/depthfinder transducer on lower outside transom and see if water starts coming out of screw holes. This can help you determine if your transom is wet. You can also buy/borrow a moisture meter to determine water content. I'm not familiar with them, but I've heard that a good meter with a knowledgeable person can be real plus in situations like yours. Consider a surveyor to check for you. You could also consider getting the original hull weight from Grady, adding engine, trailer and other bolted on parts weight, and then go to a weigh station and weigh it up. If you're not sure of trailer weight, drop boat in water and go weigh it by itself.
On the other side of the coin is the engine, and its' installation. You say it is a new 150 etec. Is it installed at the proper height? Cavitation plate (flat part directly above prop) should be even to hull bottom in front of it as a general rule of thumb. If you find that your hull is fine, and engine is at proper height, then you are either trimming motor wrong at top end, you are severely overloaded, or you are running the wrong prop. Theres a guy called "PropMaster" on the HullTruth forum that can tell you exactly the correct prop you need if you can give him weight/hp, etc. Some places that sell props will have loaners you can try, so you can zero in your manufacturers recommendations at wot. If all else checks out, then I would #1, either sand/scrape off bottom paint to gelcoat or apply new smooth bottom paint, and #2, tell dealer that sold/installed new etec to install the prop necessary to run this boat to wot at rpm recommendation provided by Evinrude. I would think as an authorized dealer, that would be their responsibility.