I'm not sure how much hill you're talking about, but trying to backup a boat with a 300hp single turbo motor is asking quite a bit. Are you sure that's all the boat/trailer weigh? I have found that people seriously underestimate the weight of what they're pulling. My Malibu Wakesetter with the trailer, full of fuel and gear on a scale is about 6500lbs.

I have a 35d which makes significantly more power and torque than your 35i and it has to work to get the boat up the slanted curb into my driveway.