Could be a lot of things.
If it's spraying water up into the boat, generally speaking it's too low on the transom. But if you trim it "out" one hole and it blows out, it could be more than one issue. Slipping hub. Prop design. Turbulent water (from imperfection in the hull?) getting to the prop. Exhaust excessive "leak" between the prop barrel and the foot. IMO it should not blow out if the AV plate is even with the bottom. I run mine about 1.5" up and I feel that it's too low, but can't go any higher.
Jack plate would help as long as the prop will let it run higher on the transom. But, I wouldn't just use a jack plate. I'd use a PT&T. Puts the motor back 5" or thereabouts. The setback alone would let you move the motor up. Plus you'll have PT&T. With PT&T you can remove the trim pin totally. Trim it all the way in, it planes almost instantly, and flat, then trim it to what your setup "likes". Best of both worlds. Normally on most of the WE boats I've messed with, PT&T alone with it's adjustment of motor height usually adds 2-3 mph without even changing the prop. Mess with the prop some and 5mph can be had-easily. Some guys get a lot more. I'd have one on mine in a heartbeat if it would fit in the garage but I don't have 5 extra inches to spare. I have about 0.5 inches to spare, if that.